ar71xx: fix sysupgrade syntax error

Fix syntax error introduced with 18832811 ("treewide: move
nand_do_upgrade call to platform_do_upgrade").

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -815,7 +815,7 @@ platform_do_upgrade() {
 	rb-911g-5hpnd|\
 	rb-912uag-2hpnd|\
 	rb-912uag-5hpnd|\
-	rb-921gs-5hpacd-r2
+	rb-921gs-5hpacd-r2|\
 	rb-951g-2hnd|\
 	rb-951ui-2hnd|\
 	rb-2011il|\