media: usb: em28xx: Replace GFP_ATOMIC with GFP_KERNEL in em28xx_init_usb_xfer()

em28xx_init_usb_xfer() is never called in atomic context.
It calls usb_submit_urb() with GFP_ATOMIC, which is not necessary.
GFP_ATOMIC can be replaced with GFP_KERNEL.

This is found by a static analysis tool named DCNS written by myself.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>