net: phy: add helper to convert negotiation result to phy settings

Add a helper to convert the result of the autonegotiation advertisment
into the PHYs speed and duplex settings.  If the result is full duplex,
also extract the pause mode settings from the link partner advertisment.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>