6in4/6in4.sh: remove some bashism (usage of [[)

"[[" is a bash extension for test. As the ash-implementation is not
fully compatible we drop its usage.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>

package/network/ipv6/6in4/files/6in4.sh

@@ -75,7 +75,7 @@ proto_6in4_setup() {
 	[ -n "$ip6addr" ] && {
 		local local6="${ip6addr%%/*}"
 		local mask6="${ip6addr##*/}"
-		[[ "$local6" = "$mask6" ]] && mask6=
+		[ "$local6" = "$mask6" ] && mask6=
 		proto_add_ipv6_address "$local6" "$mask6"
 		proto_add_ipv6_route "::" 0 "" "" "" "$local6/$mask6"
 	}