media: mt9p031: Read back the real clock rate

The real and requested clock can differ and because it is used to
calculate PLL values, the real clock rate should be read.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>