mac80211: handle S1G low rates

S1G doesn't have legacy (sband->bitrates) rates, only MCS.
For now, just send a frame at MCS 0 if a low rate is
requested. Note we also redefine (since we're out of TX
flags) TX_RC_VHT_MCS as TX_RC_S1G_MCS to indicate an S1G
MCS. This is probably OK as VHT MCS is not valid on S1G
band and vice versa.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-12-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>