net: phy: marvell: avoid pause mode on SGMII-to-Copper for 88e151x

Observed on the 88e1512 in SGMII-to-Copper mode, negotiating pause
is unreliable.  While the pause bits can be set in the advertisment
register, they clear shortly after negotiation with a link partner
commences irrespective of the cause of the negotiation.

While these bits may be correctly conveyed to the link partner on the
first negotiation, a subsequent negotiation (eg, due to negotiation
restart by the link partner, or reconnection of the cable) will result
in the link partner seeing these bits as zero, while the kernel
believes that it has advertised pause modes.

This leads to the local kernel evaluating (eg) symmetric pause mode,
while the remote end evaluates that we have no pause mode capability.

Since we can't guarantee the advertisment, disable pause mode support
with this PHY when used in SGMII-to-Copper mode.

The 88e1510 in RGMII-to-Copper mode appears to behave correctly.

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