net : phy: marvell: Speedup TDR data retrieval by only changing page once

Getting the TDR data requires a large number of MDIO bus
transactions. The number can however be reduced if the page is only
changed once. Add the needed locking to allow this, and make use of
unlocked read/write methods where needed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>