mac80211: initialize the last rx time when creating a station

should hopefully finally take care of the nasty reassociation issues which showed up as
Jan  1 00:51:10 OpenWrt daemon.info hostapd: wlan0: STA 00:xx:xx:xx:xx:xx IEEE 802.11: deauthenticated due to inactivity
right after associating, leading to an immediate disconnect.
Will probably fix #8343, #8830 and others.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@26251 3c298f89-4303-0410-b956-a3cf2f4a3e73