[media] V4L2: mt9t031: don't Oops if asynchronous probing is attempted

The mt9t031 driver hasn't yet been updated to support asynchronous
subdevice probing. If such a probing is attempted, the driver is allowed
to fail, but it shouldn't Oops. This patch fixes such a potential NULL
pointer dereference.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>