fs_enet: Update for API changes

This driver was recently broken by several changes for which this
driver was not (or was improperly) updated:

1. SET_MODULE_OWNER() was removed.
2. netif_napi_add() was only being called when building with
the old CPM binding.
3. The received/budget test was backwards.
4. to_net_dev() was wrong -- the device struct embedded in
the net_device struct is not the same as the of_platform
device in the private struct.
5. napi_disable/napi_enable was being called even when napi
was not being used.

These changes have been fixed, and napi is now on by default.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>