rt2x00: Fix TX rate short preamble detection

Mac80211 provides 2 structures to handle bitrates, namely
ieee80211_rate and ieee80211_tx_rate. To determine the short preamble
mode for an outgoing frame, the flag IEEE80211_TX_RC_USE_SHORT_PREAMBLE
must be checked on ieee80211_tx_rate and not ieee80211_rate (which rt2x00 did).

This fixes a regression which was triggered in 2.6.29-rcX as reported by Chris Clayton.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Tested-By: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>