veth: remove redundant call of dev_alloc_name

it's called in the following register_netdevice. No need to call it
here.
Tested with "ip link add type veth" and "ip link add xxx%d type veth".

Signed-off-by: Hong Zhiguo <honkiko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>