iwlwifi: dvm: remove unused variable compiler warning in debugfs.c

When compiling with W=1, we get the following warning in debugfs.c:

/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/debugfs.c: In function ‘iwl_dbgfs_fw_restart_write’:
/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/debugfs.c:2313:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
  int ret;
      ^

Fix it by ignoring the return value in this case.
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/dvm/debugfs.c

@@ -2310,7 +2310,7 @@ static ssize_t iwl_dbgfs_fw_restart_write(struct file *file,
 {
 	struct iwl_priv *priv = file->private_data;
 	bool restart_fw = iwlwifi_mod_params.restart_fw;
-	int ret;
+	int __maybe_unused ret;
 
 	iwlwifi_mod_params.restart_fw = true;