mac80211: initialize sta->last_rx in sta_info_alloc

This field is used to determine the inactivity time. When in AP mode,
hostapd uses it for kicking out inactive clients after a while. Without this
patch, hostapd immediately deauthenticates a new client if it checks the
inactivity time before the client sends its first data frame.

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