netdev/phy: Use mdiobus_read() so that proper locks are taken.

Accesses to the mdio busses must be done with the mdio_lock to ensure
proper operation.  Conveniently we have the helper function
mdiobus_read() to do that for us.  Lets use it in get_phy_id() instead
of accessing the bus without the lock held.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>