net: Provide a function to get the current MAC address

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>