mac80211: initialize last_rate for S1G STAs

mac80211: initialize last_rate for S1G STAs

last_rate is initialized to zero by sta_info_alloc(), but
this indicates legacy bitrate for the last TX rate (and
invalid for the last RX rate). To avoid a warning when
decoding the last rate as legacy (before a data frame
has been sent), initialize them as S1G MCS.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20201005164522.18069-2-thomas@adapt-ip.com
[rename to ieee80211_s1g_sta_rate_init(), seems more appropriate]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>