Make iwm(4) explicitly send multicast frames at the lowest rate, instead of

Make iwm(4) explicitly send multicast frames at the lowest rate, instead of
picking a rate from the firmware RS table. This matches what other drivers do.
Also, pass the correct Tx rate to BPF (tcpdump) for 5GHz, and don't loop over
CCK rates when building the Link-Quality command's RS table for 5GHz.
tested by myself and jcs@