arch: arm: mpu: fix variable type

Fix the type of struct field index to int, to avoid
loss of precision, in case the variable holds the value
-EINVAL (in the case of errors in MPU configuration).

Signed-off-by: Ioannis Glaropoulos <[email protected]>

Author: ioannisg

arch/arm/core/cortex_m/mpu/arm_mpu_v8_internal.h

@@ -18,7 +18,7 @@
  *        is allowed.
  */
 struct dynamic_region_info {
-	u8_t index;
+	int index;
 	struct arm_mpu_region region_conf;
 };