ath10k: setup rts/frag thresholds upon vdev creation

mac80211 configures rts/frag thresholds per-hw not
per-vif. ath10k FW expects those values to be set
per-vdev (i.e. per-vif).

ath10k should now respect rts/frag thresholds set
before a given interface was brought up.

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