mvebu: cortex-a9: add upgrade methode to nas1dual

The blamed commit adds a upgrade recipe for nas1dual to specify the
firmware partition name, but does not actually include the recipe that
will be called.

Since it previously relied on the default one, add that one.

Fixes: d21720fa90 ("mvebu: fix default partition name")
Link: https://github.com/openwrt/openwrt/pull/16704
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/mvebu/cortexa9/base-files/lib/upgrade/platform.sh

@@ -75,8 +75,8 @@ platform_do_upgrade() {
 		;;
 	iptime,nas1dual)
 		PART_NAME=firmware
+		default_do_upgrade "$1"
 		;;
-
 	linksys,wrt1200ac|\
 	linksys,wrt1900ac-v1|\
 	linksys,wrt1900ac-v2|\