net: phylink: remove pause mode ethtool setting for fixed links

Remove the ability for ethtool -A to change the pause settings for
fixed links; if this is really required, we can reinstate it later.

Andrew Lunn agrees: "So I think it is safe to not implement ethtool
-A, at least until somebody has a real use case for it."

Lets avoid making things too complex for use cases that aren't being
used.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>