media: i2c: ov5648: remove unnecessary NULL check

The "mode_index == ARRAY_SIZE(ov5648_modes)" check ensures that we
exited the loop via a break statement so we know that "mode" must
be valid.  Delete this unnecessary NULL check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>