V4L/DVB (10325): em28xx: Fix for fail to submit URB with IRQs and Pre-emption Disabled

Trace:  (Provided by Douglas)

BUG: sleeping function called from invalid context at drivers/usb/core/urb.c:558
in_atomic():0, irqs_disabled():1
Pid: 4918, comm: sox Not tainted 2.6.27.5 #1
 [<c04246d8>] __might_sleep+0xc6/0xcb
 [<c058c8b0>] usb_kill_urb+0x1a/0xd8
 [<c0488e68>] ? __kmalloc+0x9b/0xfc
 [<c0488e85>] ? __kmalloc+0xb8/0xfc
 [<c058cd5a>] ? usb_alloc_urb+0xf/0x31
 [<f8dd638c>] em28xx_isoc_audio_deinit+0x2f/0x6c [em28xx_alsa]
 [<f8dd6573>] em28xx_cmd+0x1aa/0x1c5 [em28xx_alsa]
 [<f8dd65e1>] snd_em28xx_capture_trigger+0x53/0x68 [em28xx_alsa]
 [<f8aa8674>] snd_pcm_do_start+0x1c/0x23 [snd_pcm]
 [<f8aa85d7>] snd_pcm_action_single+0x25/0x4b [snd_pcm]
 [<f8aa9833>] snd_pcm_action+0x6a/0x76 [snd_pcm]
 [<f8aa98f5>] snd_pcm_start+0x14/0x16 [snd_pcm]
 [<f8aae10e>] snd_pcm_lib_read1+0x66/0x273 [snd_pcm]
 [<f8aac5a3>] ? snd_pcm_kernel_ioctl+0x46/0x5f [snd_pcm]
 [<f8aae4a7>] snd_pcm_lib_read+0xbf/0xcd [snd_pcm]
 [<f8aad774>] ? snd_pcm_lib_read_transfer+0x0/0xaf [snd_pcm]
 [<f89feeb6>] snd_pcm_oss_read3+0x99/0xdc [snd_pcm_oss]
 [<f89fef9c>] snd_pcm_oss_read2+0xa3/0xbf [snd_pcm_oss]
 [<c064169d>] ? _cond_resched+0x8/0x32
 [<f89ff0be>] snd_pcm_oss_read+0x106/0x150 [snd_pcm_oss]
 [<f89fefb8>] ? snd_pcm_oss_read+0x0/0x150 [snd_pcm_oss]
 [<c048c6e2>] vfs_read+0x81/0xdc
 [<c048c7d6>] sys_read+0x3b/0x60
 [<c04039bf>] sysenter_do_call+0x12/0x34
 =======================

The culprit in the trace is snd_pcm_action() which invokes a spin lock
which disables pre-emption which disables an IRQ which causes the
__might_sleep() function to fail the irqs_disabled() test.  Since
pre-emption is enabled then it is safe to de-allocate the memory if
you first unlink each URB.  In this instance you are safe since
pre-emption is disabled.  If pre-emption and irqs are not disabled then
call usb_kill_urb(), else call usb_unlink_urb().

Thanks to Douglas for tracking down this bug originally!!!

[dougsland@redhat.com: Fixed codyingstyle]
Signed-off-by: Robert Krakora <rob.krakora@messagenetsystems.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>