net: macb: Fix simple typo

Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: David S. Miller <davem@davemloft.net>