rt2x00: fix use of mcs rates

In case of mcs rates txrate->idx contains the mcs index to be used for
transmission. Previously the mcs values dedicated for legacy rates where
used for mcs transmissions which resulted in the use of mcs 0 in a number
of cases (e.g. for all mcs rates >= 15 as rt2x00 does not register legacy
rates with indexes >= 15).

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