platform: change @plen field of NMPlatformIPxRoute to type guint8

On netlink layer, this field is uint8_t/uchar.

A larger (signed) plen makes no sense. Adjust the signatures
to have only guint8.

src/dhcp-manager/nm-dhcp-systemd.c

@@ -312,7 +312,8 @@ lease_to_ip4_config (const char *iface,
 				continue;
 			route.network = a.s_addr;
 
-			if (sd_dhcp_route_get_destination_prefix_length (routes[i], &plen) < 0)
+			if (   sd_dhcp_route_get_destination_prefix_length (routes[i], &plen) < 0
+			    || plen > 32)
 				continue;
 			route.plen = plen;