gpio-poweroff: Use gpiod_set_value_cansleep

The power-off call is done in a context that must be able to sleep, so
use gpiod_set_value_cansleep instead of the atomic gpiod_set_value call.

This fixes a kernel warning at shutdown when the gpio is controlled
through an IO expander for example.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>