wl1271: Use minimum rate for each band for control messages

Currently the mac80211 is not telling a hardware rate controlled driver a
rate to use for association frames etc. So to be safe, use the lowest rate
of each band for communication.

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