wifi: rtl8xxxu: Set IEEE80211_HW_SUPPORT_FAST_XMIT

According to commit 60d7900d ("wlcore: enable
IEEE80211_HW_SUPPORT_FAST_XMIT"), we can use this because all the chips
have hardware rate control.

This is one of the things mac80211 requires before it will handle MSDU
aggregation for us.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/9b59e735-6b89-a557-fafc-2da87fdd5b48@gmail.com