net80211: do not cancel scan when packet is sent.

net80211: do not cancel scan when packet is sent.

Restore pre-r300383 behavior when a frame is sent:
- stop scan;
- send frame;
- when beacon arrives and our bit in TIM is not set - restart the scan.

NOTE:
Ideally, this should introduce new interface (ieee80211_pause_anyscan());
however, since ieee80211_cancel_anyscan() is not used by drivers and only
called by ieee80211_start_pkt() the current patch overrides it's behavior
instead.

Tested with Intel 3945BG, STA mode

Reviewed by:	adrian
Differential Revision:	https://reviews.freebsd.org/D7979