net: netcp: move netcp_register_interface() to after attach module

The netcp interface is not fully initialized before attach the module
to the interface. For example, the tx pipe/rx pipe is initialized
in ethss module as part of attach(). So until this is complete, the
interface can't be registered.  So move registration of interface to
net device outside the current loop that attaches the modules to the
interface.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>