When the MII_ADVERTISE register is modified by the IOCTL handler,
phydev->advertising needs recalculating. Use the _mod_ variant of
mii_adv_to_linkmode_adv_t so that bits outside of the advertise
registers are not cleared.

Fixes: c0ec3c27 ("net: phy: Convert u32 phydev->lp_advertising to linkmode")
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>