ath9k: advertise p2p dev support when chanctx

Advertise p2p device support when ath9k loaded with
use_chanctx=1.

This will fix problem, when first interface is an AP
and next we would like to run p2p_find.
Before p2p find (scan phase) failed with EOPNOTSUPP.

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