yam: fix possible memory leak in yam_init_driver

If register_netdev(dev) fails, free_netdev(dev) needs
to be called, otherwise a memory leak will occur.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>