macb: Add support of the netpoll API

With this patch in place, I'm successfully able to use the netconsole
mechanism with the Calao USB-A9263 board, which uses the AT91SAM9263
CPU, which in terms of Ethernet controller is supported by the macb
driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[haavard.skinnemoen@atmel.com: disable_irq() -> local_irq_save()]
[haavard.skinnemoen@atmel.com: convert to net_device_ops]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>