ARM: tegra: cpuidle: remove useless initialization

dev->state_count is initialized automatically by
cpuidle_register_device().

When drv->state_count is equal to dev->state_count, no need to init
this field, so removing it.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>