net: pcs: xpcs: Convert xpcs_id to dw_xpcs_desc

A structure with the PCS/PMA MMD IDs data is being introduced in one of
the next commits. In order to prevent the names ambiguity let's convert
the xpcs_id structure name to dw_xpcs_desc. The later version is more
suitable since the structure content is indeed the device descriptor
containing the data and callbacks required for the driver to correctly set
the device up.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>