net: phy: bcm54140: apply the workaround on b0 chips

The lower three bits of the phy_id specifies the chip stepping. The
workaround is specifically for the B0 stepping. Apply it only on these
chips.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>