Live compass calibration doesn't send SET_SENSOR_OFFSETS msg for 3rd compass

[omcaree]

Live compass calibrator appears to work correctly for 3 devices (NAVIO2 with external 3DR UBlox + Compass module), when completed it pops up 3 dialogues with the offsets for each device and says they have been saved. Checking the COMPASS_OFS_* values confirms they have been updated, but the pre-arm checks still complain of "Compass not calibrated". This appears to be because the SET_SENSOR_OFFSETS message is not being sent for the 3rd device, meaning the autopilot does not commit it's device ID to eeprom and the calibration check fails on this (see lines 744-755 of AP_Compass.cpp).