net: Provide a function to get the current MAC address

The current implementation exposes the eth_device struct to code that
needs to access the MAC address.  Add a wrapper function for this to
abstract away the pointer for this operation.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/mach-au1x00/au1x00_eth.c b/arch/mips/mach-au1x00/au1x00_eth.c
index 39c5b6b..a47f088 100644
--- a/arch/mips/mach-au1x00/au1x00_eth.c
+++ b/arch/mips/mach-au1x00/au1x00_eth.c
@@ -238,7 +238,7 @@
 	}
 
 	/* Put mac addr in little endian */
-#define ea eth_get_dev()->enetaddr
+#define ea eth_get_ethaddr()
 	*mac_addr_high	=	(ea[5] <<  8) | (ea[4]	    ) ;
 	*mac_addr_low	=	(ea[3] << 24) | (ea[2] << 16) |
 		(ea[1] <<  8) | (ea[0]	    ) ;