pinctrl: at91-pio4: Fix slew rate disablement

The slew rate was enabled by default for each configuration of the
pin. In case the pin had more than one configuration, even if
we set the slew rate as disabled in the device tree, the next pin
configuration would set again the slew rate enabled by default,
overwriting the slew rate disablement.
Instead of enabling the slew rate by default for each pin configuration,
enable the slew rate by default just once per pin, regardless of the
number of configurations. This way the slew rate disablement will also
work for cases where pins have multiple configurations.

Fixes: c709135e

 ("pinctrl: at91-pio4: add support for slew-rate")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20210409082522.625168-1-tudor.ambarus@microchip.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>