Allows driver to send RTS by filling tx descriptor.

The user may want to set the rts threshold. But since we have not
been taking over rate control from mac80211 to driver by setting flag
IEEE80211_HW_HAS_RATE_CONTROL, there is nothing we can do about it.
So here just store the value, and mac80211 will tell us to use rts
protection by ieee80211_tx_info::control::use_rts.
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>