enetc: add ioctl() support for PHY-related ops

If there is an attached PHY try to handle the requested ioctl with its
handler, which allows the userspace to access PHY registers, for
example. This will make mii-diag and similar tools work.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>