net: pktgen: fix mpls reset parsing

Fix mpls list reset parsing to work as describe in
Documentation/networking/pktgen.rst:

  pgset "mpls 0"    turn off mpls (or any invalid argument works too!)

- before the patch

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	     mpls: 00000001, 00000002
	Result: OK: mpls=00000001,00000002

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ echo "mpls 0" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	     mpls: 00000000
	Result: OK: mpls=00000000

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ echo "mpls invalid" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	Result: OK: mpls=

- after the patch

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	     mpls: 00000001, 00000002
	Result: OK: mpls=00000001,00000002

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ echo "mpls 0" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	Result: OK: mpls=

	$ echo "mpls 00000001,00000002" > /proc/net/pktgen/lo\@0
	$ echo "mpls invalid" > /proc/net/pktgen/lo\@0
	$ grep mpls /proc/net/pktgen/lo\@0
	Result: OK: mpls=

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>