xtensa: userspace: fix uninitialized return values in mpu_map_region_add

Ensure that *first_idx is populated for the case of adding entries
to an empty table

Signed-off-by: Anthony Giardina <anthony.giardina@intel.com>

arch/xtensa/core/mpu.c

@@ -443,6 +443,7 @@ static int mpu_map_region_add(struct xtensa_mpu_map *map,
 
 				xtensa_mpu_entry_set(entry_slot_s, start_addr, true,
 						     access_rights, memory_type);
+				*first_idx = XTENSA_MPU_NUM_ENTRIES - 1;
 			} else {
 				/*
 				 * Populate the last two entries to indicate
@@ -459,6 +460,7 @@ static int mpu_map_region_add(struct xtensa_mpu_map *map,
 				xtensa_mpu_entry_set(entry_slot_e, end_addr, false,
 						     XTENSA_MPU_ACCESS_P_NA_U_NA,
 						     CONFIG_XTENSA_MPU_DEFAULT_MEM_TYPE);
+				*first_idx = XTENSA_MPU_NUM_ENTRIES - 2;
 			}
 
 			ret = 0;