media: ov5640: fix wrong binning value in exposure calculation

ov5640_set_mode_exposure_calc() is checking binning value but
binning value read is buggy, fix this.
Rename ov5640_binning_on() to ov5640_get_binning() as per other
similar functions.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>