pinctrl: tegra: use signed bitfields for optional fields

Optional fields are set to -1 by various preprocessor macros. Make
sure the fields can actually store them.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>