[media] adv7604: Cache register contents when reading multiple bits

When extracting multiple bits from a single register read the register
once and extract the bits on the read value.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>