mac80211: remove HW_SIGNAL_DB

Giving the signal in dB isn't much more useful to userspace
than giving the signal in unspecified units. This removes
some radiotap information for zd1211 (the only driver using
this flag), but it helps a lot for getting cfg80211-based
scanning which won't support dB, and zd1211 being dB is a
little fishy anyway.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>