6in4: don't use /dev/stdout for wget calls

Busybox ash does not implement /dev/stdout, therfore any wget output
is written into a file /dev/stdout instead of onto the standard output.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44301 3c298f89-4303-0410-b956-a3cf2f4a3e73