pinctrl: nsp: Fix potential NULL dereference

platform_get_resource() may fail and return NULL, so we should
better check it's return value to avoid a NULL pointer dereference
a bit later in the code.

This is detected by Coccinelle semantic patch.

@@
expression pdev, res, n, t, e, e1, e2;
@@

res = platform_get_resource(pdev, t, n);
+ if (!res)
+   return -EINVAL;
... when != res == NULL
e = devm_ioremap_nocache(e1, res->start, e2);

Fixes: cc4fa83f ("pinctrl: nsp: add pinmux driver support for Broadcom NSP SoC")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>