staging: et131x: Fix errors caused by phydev->addr accesses before initialisation

Fix two reported bugs, caused by et131x_adapter->phydev->addr being accessed
before it is initialised, by:

- letting et131x_mii_write() take a phydev address, instead of using the one
  stored in adapter by default. This is so et131x_mdio_write() can use it's own
  addr value.
- removing implementation of et131x_mdio_reset(), as it's not needed.
- moving a call to et131x_disable_phy_coma() in et131x_pci_setup(), which uses
  phydev->addr, until after the mdiobus has been registered.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=80751
Link: https://bugzilla.kernel.org/show_bug.cgi?id=77121


Cc: stable@vger.kernel.org
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>