microbit: BUGFIX Prevent BLE Magnetometer service from initiating compass calibration
microbit: BUGFIX Prevent BLE Magnetometer service from initiating compass
calibration

MicroBitCompass initiates a calibration procedure on demand if a heading is
requested from an uncalibrated device. The BLE MagnetometerService also
attempts to retrieve a heading periodically, but it is not good behaviour to
initiate calibration as a response to this.

This patch introduces a check such that the BLE MagnetometerService will only
attempt to read heading data from a previously calibrated compass.

markshannon/microbit-dal,markshannon/microbit-dal