[media] cx23885: video instead of vbi register used

The VID_A_GPCNT register is for video, not vbi. Read from the right
register and don't write to the video register.

Based upon Devin's initial patch made for an older kernel which I
cleaned up and rebased. Thanks to Kernel Labs for that work.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>