wl12xx: set supported_rates after association

Instead of looking for supported_rates change on every tx packet,
just extract the supported_rates after association completes (station only).

Remove wl1271.sta_rate_set and WL1271_FLAG_STA_RATES_CHANGED which are
not used anymore.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>