wl1271: Use slow rates for association messages

While not associated, default the data rates to 1 and 2mbps, so that only
those rates will be used for association related message transfer. Once
associated, configure the full rate-set supported by the AP.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>