ethernet: un-export nvmem_get_mac_address()

nvmem_get_mac_address() is only called from of_net.c
we don't need the export.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>