net: ethernet: fec: Revert "net: ethernet: fec: Replace interrupt driven MDIO with polled IO"

This reverts commit 29ae6bd1.

The commit breaks ethernet function on i.MX6SX, i.MX7D, i.MX8MM,
i.MX8MQ, and i.MX8QXP platforms. Boot yocto system by NFS mounting
rootfs will be failed with the commit.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>