net: pcs: xpcs: provide a helper to get the phylink pcs given xpcs

Provide a helper to provide the pointer to the phylink_pcs struct
given a valid xpcs pointer. This will be necessary when we make
struct dw_xpcs private to pcs-xpcs.c

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