net: pcs: xpcs: replace open-coded mii_bmcr_encode_fixed()

We can now see that we have an open-coded version of
mii_bmcr_encode_fixed() when this is called with SPEED_1000:

        val = BMCR_SPEED1000;
        if (duplex == DUPLEX_FULL)
                val |= BMCR_FULLDPLX;

Replace this with a call to mii_bmcr_encode_fixed().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>