V4L/DVB: em28xx: fix a compilation warning

drivers/media/video/em28xx/em28xx-video.c: In function ‘vidioc_s_register’:
drivers/media/video/em28xx/em28xx-video.c:1617: warning: unused variable ‘rc’

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