ath10k: set def key idx for ibss

Some time ago there was a weird issue with AP
using wrong multicast keys and generating
corrupted traffic on 10.1 firmware.

Apparently a very similar problem applies for
IBSS-RSN on 999.999.0.636.

ath10k doesn't have IBSS-RSN enabled yet. This
patch is a prerequisite to support it.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>