rt2x00: Clear IEEE80211_TX_CTL_USE_RTS_CTS flag for RTS frame

For RTS/CTS-to-self frames the IEEE80211_TX_CTL_USE_RTS_CTS flag
should be cleared for the tx_info flags.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>