net: phy: fix flag masking in __set_phy_supported

Currently also the pause flags are removed from phydev->supported because
they're not included in PHY_DEFAULT_FEATURES. I don't think this is
intended, especially when considering that this function can be called
via phy_set_max_speed() anywhere in a driver. Change the masking to mask
out only the values we're going to change. In addition remove the
misleading comment, job of this small function is just to adjust the
supported and advertised speeds.

Fixes: f3a6bd39 ("phylib: Add phy_set_max_speed helper")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>