package/busybox: Fix check for IPv6 default route in udhcpc

The check for a default route is inverted, causing the script to wait
for the timeout even when a default IPv6 route is available. Fix this up
so that it exits early as expected.

Reported-by: Bhattiprolu RaviKumar <ravikumar.bhattiprolu@gmail.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/busybox/udhcpc.script

@@ -18,7 +18,7 @@ fi
 wait_for_ipv6_default_route() {
 	printf "Waiting for IPv6 default route to appear"
 	while [ $IF_WAIT_DELAY -gt 0 ]; do
-		if [ -z "$(ip -6 route list | grep default)" ]; then
+		if ip -6 route list | grep -q default; then
 			printf "\n"
 			return
 		fi