mfd: intel_soc_pmic_core: Terminate panel control GPIO lookup table correctly

GPIO lookup tables are supposed to be zero terminated. Let's do that
and avoid accidentally walking off the end.

Cc: stable@vger.kernel.org
Fixes: 61dd2ca2

 ("mfd: intel_soc_pmic_core: Add lookup table for Panel Control as GPIO signal")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>