nl80211: Add signal strength and bandwith to nl80211station info

This patch adds signal strength and transmission bitrate
to the station_info of nl80211.

Signed-off-by: Henning Rogge <rogge@fgan.de>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>