AP_Mount: Viewpro date fix (and other small fixes)

[rmackay9]

This makes 3 unrelated fixes to AP's camera and mount subsystems found during recent testing. These changes have been tested on real hardware.

1. Viewpro driver fix to the date sent to the camera gimbal. The mask applied to the date was incorrect meaning it was incorrect after the 15th of the month. Below is a screenshot of the Viewlink app showing the correct date after this PR was applied.
   
2. AP_Mount's DEVID field should be writeable so that users can reset it to zero in case they swap one Xacti camera for another (assuming the other has a different DroneCAN node id).
3. RC_Channel's init_aux_function was missing entries for the recently added CAMERA_IMAGE_TRACKING and CAMERA_LENS features. For image tracking we don't need to initialise the state but for set-lens we should.