power: Fix power states length

The number of states was off by one.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/subsys/power/power.c b/subsys/power/power.c
index 7f1ef62..41e40aa 100644
--- a/subsys/power/power.c
+++ b/subsys/power/power.c
@@ -12,7 +12,7 @@
 #include <power/power_state.h>
 #include "policy/pm_policy.h"
 
-#define PM_STATES_LEN (PM_STATE_SOFT_OFF - PM_STATE_ACTIVE)
+#define PM_STATES_LEN (1 + PM_STATE_SOFT_OFF - PM_STATE_ACTIVE)
 #define LOG_LEVEL CONFIG_PM_LOG_LEVEL
 #include <logging/log.h>
 LOG_MODULE_REGISTER(power);