pinctrl-st: fix of_irq_to_resource() result check

of_irq_to_resource() has recently been fixed to return negative error #'s
along with 0 in case of failure, however the ST driver still only regards
0 as failure indication -- fix it up.

Fixes: 7a4228bb ("of: irq: use of_irq_get() in of_irq_to_resource()")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>