ibm_emac: Convert to use napi_struct independent of struct net_device

Commit da3dedd9 ("[NET]: Make NAPI polling independent of struct
net_device objects.") changed the interface to NAPI polling.  Fix up
the ibm_newemac driver so that it works with this new interface.  This
is actually a nice cleanup because ibm_newemac is one of the drivers
that wants to have multiple NAPI structures for a single net_device.

Compile-tested only as I don't have a system that uses the ibm_newemac
driver.  This conversion the conversion for the ibm_emac driver that
was tested on real PowerPC 440SPe hardware.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>