V4L/DVB (13311): uvcvideo: Fix compilation warning with 2.6.32 due to type mismatch with abs()

The abs() macro has changed in 2.6.32 and returns a long instead of an
int. Fix the driver to avoid compilation warnings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>