Use software polling (PHY_POLL) to check for link state changes instead
of relying on the EMAC's hardware polling feature.  Some PHY drivers
are unable to get a functioning link because the HW polling is not
robust enough.

The EMAC is able to poll the PHY on the MDIO bus looking for link state
changes (via the Link Status bit in the Status Register at address 0x1).
When the link state changes, the EMAC triggers an interrupt and tells the
driver what the new state is.  The feature eliminates the need for
software to poll the MDIO bus.

Unfortunately, this feature is incompatible with phylib, because it
ignores everything that the PHY core and PHY drivers are trying to do.
In particular:

1. It assumes a compatible register set, so PHYs with different registers
   may not work.

2. It doesn't allow for hardware errata that have work-arounds implemented
   in the PHY driver.

3. It doesn't support multiple register pages. If the PHY core switches
   the register set to another page, the EMAC won't know the page has
   changed and will still attempt to read the same PHY register.

4. It only checks the copper side of the link, not the SGMII side.  Some
   PHY drivers (e.g. at803x) may also check the SGMII side, and
   report the link as not ready during autonegotiation if the SGMII link
   is still down.  Phylib then waits for another interrupt to query
   the PHY again, but the EMAC won't send another interrupt because it
   thinks the link is up.

Cc: stable@vger.kernel.org # 4.11.x
Tested-by: Manoj Iyer <manoj.iyer@canonical.com>
Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>