pinctrl: tegra-xusb: Don't leak configurations

The pinctrl config helpers make a separate copy of the configuration, so
callers must make sure to free any dynamically allocated memory that was
used to store it.

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