netdev: gianfar: add MII ioctl handler

This is the same kind of wrapper that can also be found in many
other network device drivers.

Tested with a freescale MPC8349E host CPU:
Toggled the interface LEDs on a DP83865 PHY.

Signed-off-by: Clifford Wolf <clifford@clifford.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>