[media] mt9p031: Power down the sensor if no supported device has been detected

The mt9p031 driver first accesses the I2C device in its .registered()
method. While doing that it first powers the device up, but if probing
fails, it doesn't power the chip back down. This patch fixes that bug.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>