Avoid returning with mutex locked.

Fixes: fa8cb644 ("[media] ov2659: Don't depend on subdev API")

Cc: "Lad, Prabhakar" <prabhakar.csengg@gmail.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>