net: phy: Add helper to convert MII ADV register to a linkmode

The phy_mii_ioctl can be used to write a value into the MII_ADVERTISE
register in the PHY. Since this changes the state of the PHY, we need
to make the same change to phydev->advertising. Add a helper which can
convert the register value to a linkmode.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>