devm_of_iomap() returns error code or valid pointer. Check its return
value with IS_ERR().

Fixes: bd312773 ("power: reset: at91-reset: use devm_of_iomap")
Reported-by: NCristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: NClaudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: NSebastian Reichel <sebastian.reichel@collabora.com>