ath9k: fix erased ieee80211_rx_status.mactime

ath9k: fix erased ieee80211_rx_status.mactime

ath9k_rx_skb_preprocess nulls rxs and the mactime is never set again -
mactime is always 0. This causes problems in IBSS mode.

ieee80211_rx_bss_info uses mactime to decide if an IBSS merge is needed.
Without this patch the merge is triggered by each beacon received.

This can be recognized by the "beacon TSF higher than local TSF - IBSS
merge with BSSID" log message accompanying each beacon.

This problem was not completely fixed in commit
a6d2055b02dde1067075795274672720baadd3ca and is not a stable kernel fix.
It is solely intended for wireless-testing.

Signed-off-by: Jan Friedrich <jft@dev2day.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>