ath9k: Fix FIF_BCN_PRBRESP_PROMISC handling

So that a new created IBSS network
doesn't break on the first scan.

It seems to Sujith and me that this
stupid code unnecessary, too.

So remove it...

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