rt2x00: provide mac80211 a suitable max_rates value

Set up max_rates and max_rate_tries with suitable values even if we do not
support the whole functionality.

As rt2800 has a global fallback table we cannot specify more then one tx rate
per frame but since the hw will try several different rates (based on the
fallback table) we should still initialize max_rates to the maximum number of
rates we are going to try. Otherwise mac80211 will truncate our reported tx
rates and the rc algortihm will end up with incorrect data choosing unsuitable
rates for tx.

This improves throughput on rt2800 devices considerable.

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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>