iwlwifi: mvm: replace RS mutex with a spin_lock

The solution with the worker still had a bug, as in order
to get sta, rcu_read_lock should be used and thus no mutex
can be used inside iwl_mvm_rs_rate_init.

Also, spin_lock is a simpler solution, no need to spawn a
dedicated worker.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>