ethernet: ehea: add missing cast

We need to cast the pointer, unlike memcpy() eth_hw_addr_set()
does not take void *. The driver already casts &port->mac_addr
to u8 * in other places.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: a96d317f

 ("ethernet: use eth_hw_addr_set()")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>