usb: gadget: function: uac2: add a release method

devices are required to provide a release method. This
patch fixes the following WARN():

[   42.611159] ------------[ cut here ]------------
[   42.616025] WARNING: CPU: 0 PID: 1453 at drivers/base/core.c:250 device_release+0x94/0xa0()
[   42.624820] Device 'snd_uac2.0' does not have a release() function, it is broken and must be fixed.
[   42.634328] Modules linked in: usb_f_uac2 g_audio(-) libcomposite configfs xhci_hcd snd_soc_davinci_mcasp snd_soc_edma snd_soc_tlv320aic3x snd_soc_omap snd_soc_evm snd_soc_core dwc3 snd_compress omapdrm snd_pcm_dmaengine snd_pcm snd_timer snd fb_sys_fops lis3lv02d_i2c matrix_keypad dwc3_omap lis3lv02d panel_dpi input_polldev soundcore
[   42.665687] CPU: 0 PID: 1453 Comm: modprobe Tainted: G      D        3.17.0-rc6-00448-g9f3d0ec-dirty #188
[   42.675756] [<c0017338>] (unwind_backtrace) from [<c0012fdc>] (show_stack+0x20/0x24)
[   42.683911] [<c0012fdc>] (show_stack) from [<c0647fbc>] (dump_stack+0x8c/0xa4)
[   42.691526] [<c0647fbc>] (dump_stack) from [<c0049950>] (warn_slowpath_common+0x7c/0xa0)
[   42.700004] [<c0049950>] (warn_slowpath_common) from [<c00499b4>] (warn_slowpath_fmt+0x40/0x48)
[   42.709194] [<c00499b4>] (warn_slowpath_fmt) from [<c0405f7c>] (device_release+0x94/0xa0)
[   42.717794] [<c0405f7c>] (device_release) from [<c032e8e8>] (kobject_cleanup+0x4c/0x7c)
[   42.726189] [<c032e8e8>] (kobject_cleanup) from [<c032e7c8>] (kobject_put+0x60/0x90)
[   42.734316] [<c032e7c8>] (kobject_put) from [<c0406320>] (put_device+0x24/0x28)
[   42.741995] [<c0406320>] (put_device) from [<c040c008>] (platform_device_unregister+0x2c/0x30)
[   42.751061] [<c040c008>] (platform_device_unregister) from [<bf2b6b70>] (afunc_unbind+0x2c/0x68 [usb_f_uac2])
[   42.761523] [<bf2b6b70>] (afunc_unbind [usb_f_uac2]) from [<bf29dbec>] (remove_config.isra.8+0xe8/0x100 [libcomposite])
[   42.772868] [<bf29dbec>] (remove_config.isra.8 [libcomposite]) from [<bf29f9a4>] (__composite_unbind+0x48/0xb0 [libcomposite])
[   42.784855] [<bf29f9a4>] (__composite_unbind [libcomposite]) from [<bf29fa28>] (composite_unbind+0x1c/0x20 [libcomposite])
[   42.796446] [<bf29fa28>] (composite_unbind [libcomposite]) from [<c04d229c>] (usb_gadget_remove_driver+0x78/0xb0)
[   42.807224] [<c04d229c>] (usb_gadget_remove_driver) from [<c04d2348>] (usb_gadget_unregister_driver+0x74/0xb8)
[   42.817742] [<c04d2348>] (usb_gadget_unregister_driver) from [<bf29db00>] (usb_composite_unregister+0x1c/0x20 [libcomposite])
[   42.829632] [<bf29db00>] (usb_composite_unregister [libcomposite]) from [<bf2b1084>] (audio_driver_exit+0x14/0x1c [g_audio])
[   42.841430] [<bf2b1084>] (audio_driver_exit [g_audio]) from [<c00c0fe0>] (SyS_delete_module+0x120/0x1b0)
[   42.851415] [<c00c0fe0>] (SyS_delete_module) from [<c000ed40>] (ret_fast_syscall+0x0/0x48)
[   42.860075] ---[ end trace bb22e678d8d6db7b ]---
root@saruman:~#

Signed-off-by: Felipe Balbi <balbi@ti.com>