media: i2c: ov5670: Implement init_cfg

Implement the .init_cfg() pad operation and initialize the default
format with the default full resolution mode 2592x1944.

With .init_cfg() pad operation implemented the deprecated .open()
internal operation can now be dropped.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>