mmc: meson-gx: Convert to pinctrl_select_default_state()

Let's drop the boilerplate code for managing the default pinctrl state and
convert into using the new pinctrl_select_default_state().

Cc: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191206170821.29711-3-ulf.hansson@linaro.org