pinctrl: tegra: Return const pointer from tegra_pinctrl_get_group()

Instead of returning const pointer from tegra_pinctrl_get_group()
the return value is being casted.

This change helps return const pointer.

Signed-off-by: Prathamesh Shete <pshete@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>