ath10k: enable IEEE80211_HW_SUPPORT_FAST_XMIT

The driver can clearly enable fast-xmit since it does rate
control in the device and thus must do duration calculation
there as well.

Acked-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>