media: ov5647: Protect s_stream() with mutex

Use the driver mutex to protect s_stream() operations.
This will become more relevant once the sensor will support more formats
and set_format() could be issue concurrently to s_stream().

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>