ARM: tegra: pm: fix build error w/o PM_SLEEP

This fixes the building error when the PM_SLEEP is disabled. The fucntional
defintion of "tegra_pm_validate_suspend_mode" without "static inline"
would become a multiple definition error.

Reported-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>