pinctrl: merrifield: Use BUFCFG_PINMODE_GPIO in ->pin_dbg_show()

Use explicit comparison to BUFCFG_PINMODE_GPIO instead of implying it.

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>