media: i2c: Provide ov7251_check_hwcfg()

Move the endpoint checking from .probe() to a dedicated function,
and additionally check that the firmware provided link frequencies
are a match for those supported by the driver. Store the index to the
matching link frequency so it can be easily identified later.

Signed-off-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>