Copter: reset ROI when requested, regardless of current MOUNT_MODE

Through a certain combination of buttons on DroidPlanner, it was possible to send a `reset ROI` (an ROI with 0,0,0) twice.  But sending a `reset ROI` if the ROI is already at the default position will cause ROI to turn on again and point to 0,0,0.  The user would then lose RC control of the gimbal.

It doesn't make sense for the GCS to need to check the ROI mode before resetting ROI.  A `reset` command should always reset, regardless of what the current mode is.