rt2x00: Fix TX short preamble detection

The short preamble mode was not correctly detected during TX,
rt2x00 used the rate->hw_value_short field but mac80211 is not
using this field that way.
Instead the flag IEEE80211_TX_RC_USE_SHORT_PREAMBLE should be
used to determine if the frame should be send out using
short preamble or not.

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