net: pcs: xpcs: add _modify() accessors

The xpcs driver does a lot of read-modify-write operations on
registers, which leads to long-winded code to read the register, check
whether the read was successful, modify the value in some way, and then
write it back.

We have a mdiodev _modify() accessor that encapsulates this, and does
the register modification under the MDIO bus lock ensuring that the
modification is atomic with respect to other bus operations. Convert
the xpcs driver to use this accessor.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>