ARM: tegra: tegra_powergate_is_powered should be static

Not exported and not used externally.

Also, fix return type. Due to new return type, errors can't be returned
so WARN_ON instead of returning error if a bad parameter is specified.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>