[FIX] account_asset: fix computation of default value

_get_asset_method returns None as it sets a value on a recordset. What we do
here is simply copying the computation directly in the default get to have a
correct value instead of None. This has probably been introduced when
migrating the module at the new API at commit 6120babb45.

addons/account_asset/wizard/account_asset_change_duration.py

@@ -54,7 +54,8 @@ class AssetModify(models.TransientModel):
         if 'method_end' in fields and asset.method_time == 'end':
             res.update({'method_end': asset.method_end})
         if self.env.context.get('active_id'):
-            res['asset_method_time'] = self._get_asset_method_time()
+            asset = self.env['account.asset.asset'].browse(self.env.context.get('active_id'))
+            res['asset_method_time'] = asset.method_time
         return res
 
     @api.multi