pinctrl: mediatek: Refine mtk_pinconf_get()

Correct cases for PIN_CONFIG_SLEW_RATE, PIN_CONFIG_INPUT_SCHMITT_ENABLE,
and PIN_CONFIG_OUTPUT_ENABLE -
Use variable ret to receive value in mtk_hw_get_value() (instead of
variable val) since pinconf_to_config_packed() at end of this function
use variable ret to pack config value.

Signed-off-by: Light Hsieh <light.hsieh@mediatek.com>
Link: https://lore.kernel.org/r/1579675994-7001-4-git-send-email-light.hsieh@mediatek.com
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>