microbit: Default NLE transmission morefor OPEN mode set to 0dbm
microbit: Default NLE transmission morefor OPEN mode set to 0dbm

Update to MicroBitConfig.h such that the MICROBIT_BLE_OPEN option now also
sets the radio transmission power to its hardware default (0dbm),
rather than the child protection setting (-30dbm).

markshannon/microbit-dal,markshannon/microbit-dal