pktgen: Fix netdevice unregister

When multi queue compatable names are used by pktgen (eg eth0@0),
we currently cannot unload a NIC driver if one of its device
is currently in use.

Allow pktgen_find_dev() to find pktgen devices by their suffix (netdev name)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>