media: mt9m111: add s_stream callback

Add callback to check if we are already streaming. Now other callbacks
can check the state and return -EBUSY if we already streaming.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>