iio: imu: st_lsm6dsx: Avoid potential array overflow in st_lsm6dsx_set_odr()

The length of hw->settings->odr_table is 2 and ref_sensor->id is an enum
variable whose value is between 0 and 5.
However, the value ST_LSM6DSX_ID_MAX (i.e. 5) is not caught properly in
 switch (sensor->id) {

If ref_sensor->id is ST_LSM6DSX_ID_MAX, an array overflow will ocurrs in
function st_lsm6dsx_check_odr():
  odr_table = &sensor->hw->settings->odr_table[sensor->id];

and in function st_lsm6dsx_set_odr():
  reg = &hw->settings->odr_table[ref_sensor->id].reg;

To avoid this array overflow, handle ST_LSM6DSX_ID_GYRO explicitly and
return -EINVAL for the default case.

The enum value ST_LSM6DSX_ID_MAX is only present as an easy way to check
the limit and as such is never used, however this is not locally obvious.
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Teng Qi <starmiku1207184332@gmail.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/20211011114003.976221-1-starmiku1207184332@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

@@ -1281,6 +1281,8 @@ st_lsm6dsx_set_odr(struct st_lsm6dsx_sensor *sensor, u32 req_odr)
 	int err;
 
 	switch (sensor->id) {
+	case ST_LSM6DSX_ID_GYRO:
+		break;
 	case ST_LSM6DSX_ID_EXT0:
 	case ST_LSM6DSX_ID_EXT1:
 	case ST_LSM6DSX_ID_EXT2:
@@ -1306,8 +1308,8 @@ st_lsm6dsx_set_odr(struct st_lsm6dsx_sensor *sensor, u32 req_odr)
 		}
 		break;
 	}
-	default:
-		break;
+	default: /* should never occur */
+		return -EINVAL;
 	}
 
 	if (req_odr > 0) {