tegra: gpio: fix null label regression

tegra: gpio: fix null label regression

Fix Tegra GPIO driver to not crash resp. misbehave upon requesting
GPIOs with an empty aka NULL label. As the driver uses exclusively the
label to check for reservation status actually supplying one is
mandatory!

This fixes a regression introduced by commit:

2fccd2d96badcdf6165658a99771a4c475586279
tegra: Convert tegra GPIO driver to use driver model

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>