iwlwifi: mvm: initialize status in iwl_mvm_add_int_sta_common()

We always need to initialize the status argument to the success case
before calling iwl_mvm_send_cmd_status() or
iwl_mvm_send_cmd_pdu_status() (which calls the former) otherwise we
may get an uninitialized value back.  In this case, we use
ADD_STA_SUCCESS as success.

Fixes: 732d06e9 ("iwlwifi: mvm: add station before allocating a queue")
Reported by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>