[ARM] 3342/1: NSLU2: Protect power button init routine with machine_is_nslu2()

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: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>