net: arc_emac: Add support for ndo_do_ioctl net_device_ops operation

This operation is required for handling ioctl commands like SIOCGMIIREG,
when debugging MDIO registers from userspace.

This commit adds support for this operation.

Signed-off-by: Romain Perier <romain.perier@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>