V4L/DVB: video: testing unsigned for less than 0

soc_mbus_bytes_per_line() returns -EINVAL on error but we store it in an
unsigned int so the test for less than zero doesn't work.  I think it
always returns "small" positive values so we can just cast it to int
here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>