simtech: fix updating bitmask during gps disabling

During disabling of gps sources the bitmask that keeps track of them is updated incorrectly. Instead of removing the current source, all other sources are removed from the mask.

One problem that arises from this is, that when you enable GPS after it has been disabled completely (e.g. by disabling all GPS sources), the code will not send a "+CGPS=1,1" command because it incorrectly assumes that GPS is still enabled on the device.