media: dvb-frontends: rtl2832_sdr: Replace GFP_ATOMIC with GFP_KERNEL

rtl2832_sdr_submit_urbs(), rtl2832_sdr_alloc_stream_bufs(), and
rtl2832_sdr_alloc_urbs() are never called in atomic context.
They call usb_submit_urb(), usb_alloc_coherent() and usb_alloc_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>