V4L/DVB: ivtv, ivtvfb: Use a define for the output line and field register address

Get rid of the magic number 0x28c0 used in several places in the ivtv and
ivtvfb driver and define the register address to a meaningful name.

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