[media] ivtv-ioctl.c: remove an useless check

drivers/media/pci/ivtv/ivtv-ioctl.c: In function 'ivtv_s_input':
drivers/media/pci/ivtv/ivtv-ioctl.c:996:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>