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/powerpc/cpu/mpc85xx/ether_fcc.c b/arch/powerpc/cpu/mpc85xx/ether_fcc.c
index 166dc9e..58d4bfb 100644
--- a/arch/powerpc/cpu/mpc85xx/ether_fcc.c
+++ b/arch/powerpc/cpu/mpc85xx/ether_fcc.c
@@ -338,7 +338,7 @@
      * it unique by setting a few bits in the upper byte of the
      * non-static part of the address.
      */
-#define ea eth_get_dev()->enetaddr
+#define ea eth_get_ethaddr()
     pram_ptr->fen_paddrh = (ea[5] << 8) + ea[4];
     pram_ptr->fen_paddrm = (ea[3] << 8) + ea[2];
     pram_ptr->fen_paddrl = (ea[1] << 8) + ea[0];