power: reset: at91-poweroff: do not procede if at91_shdwc is allocated

power: reset: at91-poweroff: do not procede if at91_shdwc is allocated

[ Upstream commit 9f1e44774be578fb92776add95f1fcaf8284d692 ]

There should be only one instance of struct shdwc in the system. This is
referenced through at91_shdwc. Return in probe if at91_shdwc is already
allocated.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>