network: fix error aggregation in wwan_check_and_set_configuration()

When removing marked routes, the condition `if (ret)` incorrectly
overwrites any previously accumulated error in `ret` with the latest
return value `r`, even if `r >= 0` (success). This means an earlier
real error can be silently cleared by a subsequent successful
route_remove() call.

The parallel address_remove() block just above uses the correct
`if (r < 0)` pattern. Apply the same fix to the route_remove() block.

src/network/networkd-wwan.c

@@ -529,7 +529,7 @@ static int link_apply_bearer_impl(Link *link, Bearer *b) {
                         continue;
 
                 r = route_remove(route, link->manager);
-                if (ret)
+                if (r < 0)
                         ret = r;
         }