media: i2c: ov8865: remove unnecessary NULL check

The check on mode_index is sufficient to ensure that we have a
valid mode. Remove the explicit mode check similarly to
commit 38a50230 ("media: i2c: ov5648: remove unnecessary NULL check")

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>