pktgen: correctly handle failures when adding a device

The return value of pktgen_add_device() is not checked, so
even if we fail to add some device, for example, non-exist one,
we still see "OK:...". This patch fixes it.

After this patch, I got:

	# echo "add_device non-exist" > /proc/net/pktgen/kpktgend_0
	-bash: echo: write error: No such device
	# cat /proc/net/pktgen/kpktgend_0
	Running:
	Stopped:
	Result: ERROR: can not add device non-exist
	# echo "add_device eth0" > /proc/net/pktgen/kpktgend_0
	# cat /proc/net/pktgen/kpktgend_0
	Running:
	Stopped: eth0
	Result: OK: add_device=eth0

(Candidate for -stable)

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>