media: dvb-usb-v2: stop using coherent memory for URBs

There's no need to use coherent buffers there. So, let the
DVB core do the allocation. That should give some performance
gain outside x86.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>