net: pcs: xpcs: use FIELD_PREP() and FIELD_GET()

Convert xpcs to use the bitfield macros rather than definining the
bitfield shifts and open-coding the insertion and extraction of these
bitfields.

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