Undo supposed 'low battery' status changes.

The commits 0eaf9dd4a5 and 5142f38708 were based on a misunderstanding
about bit 7 of the 'wind_dir' byte. This commit corrects that mistake
and adds an 'invalid wind dir' status bit. If you reprocessed your data
after commit 5142f38708 you need to reprocess it again. Otherwise,
reprocessing is optional.