net: pcs: xpcs: move definition of struct dw_xpcs to private header

There should be no reason for anything outside the XPCS code to know
the contents of struct dw_xpcs - this is a private structure to XPCS.
Move the definition to the private pcs-xpcs.h header, leaving a
declaration in the global pcs/pcs-xpcs.h

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