iwlwifi: mvm: handle device start failure correctly

If the device fails to start correctly prior to loading the
regular runtime firmware (after having run the INIT firmware),
treat that error correctly by actually checking the return
value of _iwl_trans_start_hw() and stopping the device again
before returning an error.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>