iwlwifi: mvm: hold mutex when flushing in iwl_mvm_flush_no_vif()

The iwl_mvm_flush_tx_path() function sends a synchronous command to
the firmware.  When doing that, we must hold the mutex.  The
iwl_mvm_flush_no_vif() function was mistakenly not holding the mutex.
Fix it.

Fixes: 6110d9e5 ("iwlwifi: mvm: Flush non STA TX queues")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>