pinctrl: intel: merrifield: Add missed check in mrfld_config_set()

Not every pin can be configured. Add missed check to prevent access
violation.

Fixes: 4e80c8f5 ("pinctrl: intel: Add Intel Merrifield pin controller support")
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>