ath9k: setup rxfilter when offchannel

Setup rxfiler correctly for offchannel ctx.

This fix problem we didn't configure rxfilter, next
didn't receive probe requests and next failed
p2p_find. This was seen when ath9k loaded with
use_chanctx=1

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>