arm/mach-at91/pm : fix possible object reference leak

of_find_device_by_node() takes a reference to the struct device
when it finds a match via get_device. When returning error we should
call put_device.

Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>