netdev/phy/icplus: Use mdiobus_write() and mdiobus_read() for proper locking.

Usually you have to take the bus lock.  Why not here too?

I saw this when working on something else.  Not even compile tested.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Cc: "Uwe Kleine-Konig" <u.kleine-koenig@pengutronix.de>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>