ath10k: enable VHT for IBSS

Enable VHT support for IBSS, while mac80211/cfg80211 and
wpa_supplicant already support this.

In my test env, qca988x 2x2 I get:

(udp) ath10k-1 >>>> ath10k-2 (server) -  speed: 419 Mbits/sec
(tcp) ath10k-1 >>>> ath10k-2 (server) -  speed: 404 Mbits/sec

During tests I used wpa_supplicant (latest git version), which
already support IBSS VHT, and choose highest available BW. Also tested with
qca6174.

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