ath9k: Don't reset TSF after scanning automatically

Reset automatically the TSF on re-enabling beaconing after scanning
in IBSS mode causes several problems. For example a new created IBSS
network can't age before an other node has joined, because scans are
done automatically in that case. And several other strange bugs more...

The TSF reset is done manually in the higher level mac80211 code in
the cases were it's needed, so we don't need to do it here.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>