mwl8k: honor peer rate set

When calling SET_RATE, SET_AID, or when creating a station database
entry for our AP, pass in the AP's rate set instead of just blindly
enabling all legacy rates, so as to end up doing the right thing when
talking to 11b-only APs.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>