ethernet: arc: remove use of 'struct platform_device'

This is a preparation of an api changes for the emac_main.c module.
The involved functions are arc_emac_probe and arc_emac_remove.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>