Correctly update the extender's label when the battery is unplugged

This commit is dedicated to my beloved notebook luna, which fortunately didn't die last night after the resident cat poured half a glass of water into it.

I'm not a 100% sure this commit fixes the issue fully, but HAL decided to report two batteries after some un- and replugging, so it's pretty hard to test until that goes back to normal (maybe another glass of water is needed!?).

This patch does improve updating the extender's label however, so there you go.