writing to this register could trigger a FEC_ENET_MII interrupt actually
creating a race condition with fec_enet_mdio_read() that is called on
link change also.

Another point to consider:

static int fec_enet_mdio_wait(struct fec_enet_private *fep)
{
        uint ievent;
        int ret;

        ret = readl_poll_timeout_atomic(fep->hwp + FEC_IEVENT, ievent,
                                        ievent & FEC_ENET_MII, 2, 30000);

        if (!ret)
                writel(FEC_ENET_MII, fep->hwp + FEC_IEVENT);

        return ret;
}

An unexpected interrupt will make this exit too early, and the read
will get invalid data. An unexpected interrupt would not cause a
timeout here, which is what you are reporting.

	Andrew