pinctrl: baytrail: Do not call WARN_ON for a firmware bug

WARN_ON causes a backtrace to get logged which is only useful for
kernel bugs. For signalling a firmware bug dev_warn(dev, FW_BUG "...")
should be used.

This fixes users running userspace software to monitor kernel oopses
getting a false positive bug-report every boot because of the wrong
use of WARN_ON.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>