media: ov8856: Check reading clock frequency succeeded

Instead of blindly trusting getting the clock frequency succeeded end then
testing it against a pre-defined value, verify reading the value
succeeded.

Fixes: 879347f0 ("media: ov8856: Add support for OV8856 sensor")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>