NET: sb1250-mac: Add missing MODULE_LICENSE()

As per comment, the code has always been GPLv2 licensed.

This fixes the follwoing modpost warning:

WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/broadcom/sb1250-mac.o
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

drivers/net/ethernet/broadcom/sb1250-mac.c

@@ -2641,3 +2641,4 @@ static struct platform_driver sbmac_driver = {
 };
 
 module_platform_driver(sbmac_driver);
+MODULE_LICENSE("GPL");