media: ov5647: Rename SBGGR8 VGA mode

Before adding new modes, rename the only existing one to report
the bit depth to distinguish it from future additions.

While at it, briefly describe the mode.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>