rt2x00: Don't call ieee80211_get_tx_rate for MCS rates

ieee80211_get_tx_rate is not valid for HT rates. Hence, restructure the
TX desciptor creation to be aware of MCS rates. The generic TX desciptor
creation now cares about the rate_mode (CCK, OFDM, MCS, GF).

As a result, ieee80211_get_tx_rate gets only called for legacy rates.

Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>