ethernet: broadcom/sb1250-mac: clean up after SIBYTE_BCM1x55 removal

With commit b984d7b5

 ("MIPS: sibyte: Remove unused config option
SIBYTE_BCM1x55"), some #if's in the Broadcom SiByte SOC built-in Ethernet
driver can be simplified.

Simplify prepreprocessor conditions after config SIBYTE_BCM1x55 removal.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>