iwm(4) should always send multicast frames at the lowest rate.
We probably got lucky and frames were still sent at a compatible rate
via the LQ retry table. But it is better to have an "IS_MULTICAST" check
like other drivers do.
On 5GHz, iwm(4) passes the wrong rate to BPF. This is a cosmetic issue.
E.g. "tcpdump -n -i iwm0 -y IEEE802_11 -v" will show "1 Mbit/s" for
multicast frames, which is wrong (should be 6Mbit/s on 5Ghz).
In iwm_setrates(), don't bother looping over CCK rates if we're on a 5 GHz
channel. CCK rates aren't valid on 5GHz channels. We didn't add them, so
this is not a bug, but we might as well exit the loop early.