mac80211: fix station rate table updates on assoc

If the driver uses .sta_add, station entries are only uploaded after the sta
is in assoc state. Fix early station rate table updates by deferring them
until the sta has been uploaded.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210201083324.3134-1-nbd@nbd.name
[use rcu_access_pointer() instead since we won't dereference here]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>