phylib: Modify Vitesse RGMII skew settings

The Vitesse driver was using the RGMII_ID interface type to determine if
skew was necessary.  However, we want to move away from using that
interface type, as it's really a property of the board's PHY connection.
However, some boards depend on it, so we want to support it, while
allowing new boards to use the more flexible "fixups" approach.  To do
this, we extract the code which adds skew into its own function, and
call that function when RGMII_ID has been selected.

Another side-effect of this change is that if your PHY has skew set
already, it doesn't clear it.  This way, the fixup code can modify the
register without config_init then clearing it.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>