V4L/DVB (12740): em28xx: better describe vinctrl registers

Properly document the video input control register, in preparation for the
addition of VBI support.  Note this patch makes no functional changes.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>