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

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

commit 19b26d92dfb70f56440c187a20c49102ab648b97 upstream.

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

Fixes: 4e80c8f50574 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>