pinctrl: meson: Rework enable/disable bias part

rework bias enable/disable part to prepare drive-strength integration
no functional changes

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>