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_emac driver so that it works with this new interface.  This is
actually a nice cleanup because ibm_emac is one of the drivers that
wants to have multiple NAPI structures for a single net_device.

Tested with the internal MAC of a PowerPC 440SPe SoC with an AMCC
'Yucca' evaluation board.

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