cpuidle: use BIT() for idle state flags and remove CPUIDLE_DRIVER_FLAGS_MASK

Use BIT() macro to do a small tidy-up.

CPUIDLE_DRIVER_FLAGS_MASK is not used, so remove it.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>