rtl8xxxu: Obtain RTS rates from mac80211

Use the mac80211 provided rate for RTS rather than the hard coded
24Mbps as suggested by the vendor drivers.

Reported-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>