iwlwifi: mvm: add a wrapper around rs_tx_status to handle locks

iwl_mvm_rs_tx_status can be called from two places in the code, but the
mutex is taken only on one of the calls. Split it into a wrapper taking
locks and an internal __iwl_mvm_rs_tx_status function.

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>