net: mvneta: implement ->ndo_do_ioctl() to support PHY ioctls

This commit implements the ->ndo_do_ioctl() operation so that the
PHY-related ioctl() calls can work from userspace, which allows
applications like mii-tool or mii-diag to do their job.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>