net: microchip: lan743x: add fixed phy unregister support

When operating in fixed phy mode and if there is repeated open/close
phy test cases, everytime the fixed phy is registered as a new phy
which leads to overrun after 32 iterations. It is solved by adding
fixed_phy_unregister() in the phy_close path.

In phy_close path, netdev->phydev cannot be used directly in
fixed_phy_unregister() due to two reasons,
    - netdev->phydev is set to NULL in phy_disconnect()
    - fixed_phy_unregister() can be called only after phy_disconnect()
So saving the netdev->phydev in local variable 'phydev' and
passing it to phy_disconnect().

Signed-off-by: Pavithra Sathyanarayanan <Pavithra.Sathyanarayanan@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>