plugin-base: don't assume a plugin will give as supported all ports of the device

E.g. the Generic plugin will give as unsupported 'net' ports found in the
device, as it doesn't know how to use them.

src/mm-plugin-base.c

@@ -780,8 +780,13 @@ create_modem (MMPlugin  *self,
 
         key = get_key (subsys, name);
         probe = g_hash_table_lookup (priv->tasks, key);
-        g_assert (probe);
-        probes = g_list_prepend (probes, g_object_ref (probe));
+        if (!probe)
+            mm_warn ("(%s/%s) Ignoring port when creating modem with plugin '%s'",
+                     subsys,
+                     name,
+                     priv->name);
+        else
+            probes = g_list_prepend (probes, g_object_ref (probe));
         g_free (key);
     }