net: macb: kill useless use of list_empty()

The list_for_each_entry() macro already handles the case where the list
is empty (by not executing the loop body).  It's not necessary to handle
this case specially, so stop doing so.

Cc: Rafal Ozieblo <rafalo@cadence.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Julia Cartwright <julia@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>