iwlwifi: don't treat a bool as an error code

iwl_trans_grab_nic_access returns a boolean. So ret should
explicitely set to an error code and not rely on the value
returned by iwl_trans_grab_nic_access.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>