media: ov7740: enable to get exposure control in autoexposure mode

The exposure control is clustered with the autoexposure control and
flagged as volatile, but the g_volatile_ctrl() doesn't handle
V4L2_CID_EXPOSURE_AUTO.  So, the value of the exposure control can't be
read in autoexposure mode.

This enables to get the exposure control in autoexposure mode by making
ov7740_get_volatile_ctrl() deal with V4L2_CID_EXPOSURE_AUTO.

This also sets the exposure control as volatile by specifying the
argument to v4l2_ctrl_auto_cluster() instead of manually flagging it.

Cc: Wenyou Yang <wenyou.yang@microchip.com>
Cc: Eugen Hristev <eugen.hristev@microchip.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>