net: net_netdev: Fix error handling in ntb_netdev_init_module()

The ntb_netdev_init_module() returns the ntb_transport_register_client()
directly without checking its return value, if
ntb_transport_register_client() failed, the NTB client device is not
unregistered.

Fix by unregister NTB client device when ntb_transport_register_client()
failed.

Fixes: 548c237c

 ("net: Add support for NTB virtual ethernet device")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>