soc: tegra: Fix tegra_pmc_get_suspend_mode definition

When CONFIG_PM_SLEEP is disabled, the function is not defined,
causing a link failure:

arm-linux-gnueabi-ld: drivers/cpuidle/cpuidle-tegra.o: in function `tegra_cpuidle_probe':
cpuidle-tegra.c:(.text+0x24): undefined reference to `tegra_pmc_get_suspend_mode'

Change the #ifdef check according to the definition.

Fixes: 382ac8e2 ("cpuidle: tegra: Disable CC6 state if LP2 unavailable")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>