cfg80211: add a field for the bitrate of the last rx data packet from a station

Also fix a typo in the STATION_INFO_TX_BITRATE description

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

Conflicts:
	include/linux/nl80211.h
	include/net/cfg80211.h
	net/wireless/nl80211.c