macb: Fix section mismatch and shrink runtime footprint

macb devices are only found integrated on SoCs, so they can't be
hotplugged. Thus, the probe() and exit() functions can be __init and
__exit, respectively. By using platform_driver_probe() instead of
platform_driver_register(), there won't be any references to the
discarded probe() function after the driver has loaded.

This also fixes a section mismatch due to macb_probe(), defined as
__devinit, calling macb_get_hwaddr, defined as __init.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>