[media] adv7842: pixelclock read-out

Incorrect registers used for pixelclock read-out.
Same registers as for adv7604 which actually gave an almost
correct read-out, even they are not documented for adv7842.
Corrected deep-color pixel-clock correction.

Signed-off-by: Martin Bugge <marbugge@cisco.com>
Cc: Mats Randgaard <matrandg@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>