media: i2c: Remove per-mode frequencies from ov7251

Each of the defined modes in the ov7251 driver uses the same link
frequency and pixel rate; just drop those members of the modes and
set the controls to read only during initialisation. Add a new
table defining the supported pixel rates to substitue for the values
hardcoded in the modes.

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>