power: reset: at91-reset: check properly the return value of devm_of_iomap

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: Cristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>