media: em28xx-audio: use GFP_KERNEL for memory allocation during init

As far as I can tell em28xx_audio_urb_init() is called once during
initialization from non atomic context. Memory allocation from
non atomic context should use GFP_KERNEL to avoid using emergency pool
for memory allocation.
Use GFP_KERNEL for memory allocation.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>