V4L/DVB: mx2_camera: fix a race causing NULL dereference

The mx25_camera_irq irq handler may get called after the camera has been
deactivated (from mx2_camera_deactivate). Detect this situation, and bail out.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>