net: phylink: avoid reducing support mask

Avoid reducing the support mask as a result of the interface type
selected for SFP modules, or when setting the link settings through
ethtool - this should only change when the supported link modes of
the hardware combination change.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>