pinctrl: st: Fix error check for of_irq_to_resource usage

This patch fixes an error check while using of_irq_to_resource.
of_irq_to_resource returns non-zero interrupt number on success and zero
on error. The driver was using error check is wrong way.

Without this patch the driver will configure interrupt zero if there is
no interrupt specified in the node.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>