MicroBitBLEManager::restartInBLEMode() uses the wrong size for RebootModeValue

[martinwork]

On branch "code-review"...

RebootMode is deleted when it is NULL. When it's non-NULL, RebootMode leaks, though this doesn't matter much as the next line is a reset.

sizeof(RebootMode) should be sizeof(RebootModeValue).

[martinwork]

Just realised microbit_free checks for a NULL pointer, so in normal circumstances it won't crash, but deleting a NULL pointer still seems wrong!

And sizeof(RebootMode) should be sizeof(RebootModeValue).