net: phy: micrel: Fix concurrent register access

Make Extended page register accessing atomic,
to overcome unexpected output from register
reads/writes.

Fixes: 7c2dcfa2 ("net: phy: micrel: Add support for LAN8804 PHY")
Signed-off-by: Divya <Koppera&lt;Divya.Koppera@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>