rtlwifi: enable mac80211 fast-tx support

Enable the mac80211 fast-tx feature, since our driver already support
hw_flags required by fast-tx and is able to let mac80211 stack to transmit
packet through fast-xmit path.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>