pinctrl: amd: isp411: Fix IS_ERR() vs NULL check in probe()

The platform_get_resource() returns NULL on error.  It doesn't
return error pointers.  Fix the error checking to match.

Fixes: e97435ab ("pinctrl: amd: isp411: Add amdisp GPIO pinctrl")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Link: https://lore.kernel.org/617f4c77-7837-4e24-9f4d-620ecfedf924@stanley.mountain


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>