kirkwood: define empty CONFIG_MVGBE_PORTS by default

Each board with defines it's own set of values. If we do not define
CONFIG_MVGBE_PORTS we will hit following error:

mvgbe.c: In function 'mvgbe_initialize':
mvgbe.c:700:34: error: 'CONFIG_MVGBE_PORTS' undeclared (first use in this function)
  u8 used_ports[MAX_MVGBE_DEVS] = CONFIG_MVGBE_PORTS;

This patch fixes above described problem.

Signed-off-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>

drivers/net/mvgbe.c

@@ -35,6 +35,10 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+#ifndef CONFIG_MVGBE_PORTS
+# define CONFIG_MVGBE_PORTS {0, 0}
+#endif
+
 #define MV_PHY_ADR_REQUEST 0xee
 #define MVGBE_SMI_REG (((struct mvgbe_registers *)MVGBE0_BASE)->smi)