staging: bcm2835-camera: Initialise dev in v4l2_dev

In the Linux kernel, the following vulnerability has been resolved:

staging: bcm2835-camera: Initialise dev in v4l2_dev

Commit 42a2f6664e18 ("staging: vc04_services: Move global g_state to

vchiq_state") changed mmal_init to pass dev->v4l2_dev.dev to

vchiq_mmal_init, however nothing iniitialised dev->v4l2_dev, so we got

a NULL pointer dereference.

Set dev->v4l2_dev.dev during bcm2835_mmal_probe. The device pointer

could be passed into v4l2_device_register to set it, however that also

has other effects that would need additional changes.