usb: gadget: remove __devexit in f_uac2

avoids the following section missmatch
|WARNING: drivers/usb/gadget/g_audio.o(.init.text+0x2e7): Section
|mismatch in reference from the function afunc_bind() to the function
|.devexit.text:snd_uac2_remove()
|The function __init afunc_bind() references
|a function __devexit snd_uac2_remove().
|This is often seen when error handling in the init function
|uses functionality in the exit path.
|The fix is often to remove the __devexit annotation of
|snd_uac2_remove() so it may be used outside an exit section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>