HID: fix lock imbalance in hiddev

Don't forget to unlock_kernel() in hiddev_ioctl_usage().
Added in 7961df16
(HID: Switch hiddev to unlocked_ioctl).

Corresponing sparse warning:
drivers/hid/usbhid/hiddev.c:515:10: warning: context imbalance in 'hiddev_ioctl_usage': wrong count at exit
drivers/hid/usbhid/hiddev.c:515:10:    context 'kernel_lock': wanted 0, got 1

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>