[media] adv7842: corrected setting of cp-register 0x91 and 0x8f

Bit 6 of register 0x8f was cleared incorrectly (must be 1), and bit 4
of register 0x91 was set incorrectly (must be 0).

These bits are undocumented, so we shouldn't modify them to values different
from what the datasheet specifies.

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