[media] mt9v032: do not clear reserved bits in read mode register

The read mode register bits 8 and 9 are set and marked as reserved.
Don't clear them.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>