rt2x00: Don't recalculate HT40 compensation for each rate

Previously the HT40 tx power compensation value was calculated for each
rate. However, the calculation is independent of the tx rate and as such
can be precalculated and just passed in for each rate.

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>