net: mvpp2: deny disabling autoneg for 802.3z modes

The documentation for Armada 8040 says:

  Bit 2 Field InBandAnEn In-band Auto-Negotiation enable. ...
  When <PortType> = 1 (1000BASE-X) this field must be set to 1.

We presently ignore whether userspace requests autonegotiation or not
through the ethtool ksettings interface. However, we have some network
interfaces that wish to do this. To offer a consistent API across
network interfaces, deny the ability to disable autonegotiation on
mvpp2 hardware when in 1000BASE-X and 2500BASE-X.

This means the only way to switch between 2500BASE-X and 1000BASE-X
on SFPs that support this will be:

 # ethtool -s ethX advertise 0x20000006000 # 1000BASE-X Pause AsymPause
 # ethtool -s ethX advertise 0xe000        # 2500BASE-X Pause AsymPause

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Marek Behún <kabel@kernel.org>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>