watchdog: at91rm9200: Correct check for syscon_node_to_regmap() errors

syscon_node_to_regmap() returns a regmap or an ERR_PTR().

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>