net: pcs: xpcs: rename mdio_xpcs_args to dw_xpcs

The struct mdio_xpcs_args is reminiscent of when a similarly named
struct mdio_xpcs_ops existed. Now that that is removed, we can shorten
the name to dw_xpcs (dw for DesignWare).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>