media: mt9m111: add streaming check to set_fmt

Currently set_fmt don't care about the streaming status, so the format
can be changed during streaming. This can lead into wrong behaviours.

Check if the device is already streaming and return -EBUSY to avoid
wrong behaviours.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>