wl1271: Fix warning about unsupported RX rate

While scanning, it is possible that beacon and probe response frames are
received on other band than configured to the driver. In rx status
handling this has caused "Unsupported RX rate from HW" warnings. This
patch changes the wl1271_rate_to_index function to take the band of the
received frame as a parameter instead of using value configuret to
wl->band.

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>