Patch from Alessandro Zummo

The power button exit routine for the Linksys NSLU2 was not protected by
a machine_is_nslu2(). This patch fixes it.
Signed-off-by: NRod Whitby <rod@whitby.id.au>
Signed-off-by: NAlessandro Zummo <a.zummo@towertech.it>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>