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: NGregory Greenman <gregory.greenman@intel.com>
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>