rt2x00: Fix basic rate initialization

The basic rate which is configured in the register
should not match all supported rates, but only the _basic_ rates.

Fix this by adding a new flag to the rt2x00_rate structure
and whenever the mode is changed, loop over all available rates
for that band to get the basic rate mask.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>