[media] cx25821-video: remove maxw from cx25821_vidioc_try_fmt_vid_cap

After cx25821-video cleanup, this var is not used anymore:

drivers/media/pci/cx25821/cx25821-video.c: In function 'cx25821_vidioc_try_fmt_vid_cap':
drivers/media/pci/cx25821/cx25821-video.c:591:15: warning: variable 'maxw' set but not used [-Wunused-but-set-variable]

as the code now checks the max width as the default case for the
range check.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>