gpio: tegra: Use unsigned int where possible

In most of the cases, integers in this file can't be negative, so the
type can be more restricted for clarity.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>