cfg80211: allow CFG80211_SIGNAL_TYPE_UNSPEC in station_info

The station_info struct had demanded dBm signal values, but the
cfg80211 wireless extensions implementation was also accepting
"unspecified" (i.e. RSSI) unit values while the nl80211 code was
completely unaware of them.  Resolve this by formally allowing the
"unspecified" units while making nl80211 ignore them.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>