iwlwifi: mvm: avoid races in rate init and rate perform

Rate perform uses the lq_sta table to calculate the next rate to scale
while rate init resets the same table,

Rate perform is done in soft irq context in parallel to rate init
that can be called in case we are doing changes like AP changes BW
or moving state for auth to assoc.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>