rt2x00: Limit txpower by eeprom values

Limit the txpower per rate by the approriate values in the eeprom.
This avoids too high txpower values resulting in bad tx performance.

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>