mac80211: IBSS join rework

I hold back this patch for around a week to avoid
confusion. This is the second step of
"mac80211: Fixed BSSID handling revisited".

With it, in the situation of a strange merge to the
same BSSID (e.g. caused by a TSF overflow) only
reset_tsf() is called.

And sta_info_flush_delayed() is only called if you
change the network manually, not on an automatic
BSSID merge.

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