media: ov772x: use v4l2_ctrl to get current control value

The ov772x driver provides three V4L2 controls and the current value of
each control is saved as a variable in the private data structure.

We don't need to keep track of the current value by ourself, if we use
v4l2_ctrl returned from v4l2_ctrl_new_std() instead.

This is a preparatory change to avoid accessing registers under power
saving mode.  This simplifies s_ctrl() by making it just return without
saving the current control value in private area when it is called under
power saving mode.

Cc: Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>