net: phy: fix wrong mask to phy_modify()

When forcing a specific link mode, the PHY driver must clear the
existing speed and duplex bits in BMCR while preserving some other
control bits. This logic was accidentally inverted with the introduction
of phy_modify().

Fixes: fea23fb5

 ("net: phy: convert read-modify-write to phy_modify()")
Signed-off-by: Ingo van Lil <inguin@gmx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>