fix(stm32mp1): properly check PSCI functions return

The psci_get_pstate_* helpers return unsigned int values,
update the code accordingly. Remove the useless pstate variable.
This corrects MISRA C2012-14.4:
The controlling expression of an if statement and the controlling
expression of an iteration-statement shall have essentially Boolean
type.

Change-Id: Idc7e756f4ba2bc0d66a327763013f77f86fe16b2
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/st/stm32mp1/stm32mp1_pm.c b/plat/st/stm32mp1/stm32mp1_pm.c
index 8e1c1cf..ff2218f 100644
--- a/plat/st/stm32mp1/stm32mp1_pm.c
+++ b/plat/st/stm32mp1/stm32mp1_pm.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2023, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2023, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -161,17 +161,15 @@
 static int stm32_validate_power_state(unsigned int power_state,
 				      psci_power_state_t *req_state)
 {
-	int pstate = psci_get_pstate_type(power_state);
-
-	if (pstate != 0) {
+	if (psci_get_pstate_type(power_state) != 0U) {
 		return PSCI_E_INVALID_PARAMS;
 	}
 
-	if (psci_get_pstate_pwrlvl(power_state)) {
+	if (psci_get_pstate_pwrlvl(power_state) != 0U) {
 		return PSCI_E_INVALID_PARAMS;
 	}
 
-	if (psci_get_pstate_id(power_state)) {
+	if (psci_get_pstate_id(power_state) != 0U) {
 		return PSCI_E_INVALID_PARAMS;
 	}