iio:chemical:sps30 Supress some switch fallthrough warnings.

Fixes warnings reported on linux-next but marking one path
and adding an explicit return in the other.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Andreas Brauchli <a.brauchli@elementarea.net>
Acked-by: Tomasz Duszynski <tduszyns@gmail.com>

drivers/iio/chemical/sps30.c

@@ -118,6 +118,7 @@ static int sps30_do_cmd(struct sps30_state *state, u16 cmd, u8 *data, int size)
  case SPS30_READ_AUTO_CLEANING_PERIOD:
  buf[0] = SPS30_AUTO_CLEANING_PERIOD >> 8;
  buf[1] = (u8)SPS30_AUTO_CLEANING_PERIOD;
+ /* fall through */
  case SPS30_READ_DATA_READY_FLAG:
  case SPS30_READ_DATA:
  case SPS30_READ_SERIAL:
@@ -295,6 +296,8 @@ static int sps30_read_raw(struct iio_dev *indio_dev,
  *val2 = 10000;
 
  return IIO_VAL_INT_PLUS_MICRO;
+ default:
+ return -EINVAL;
  }
  default:
  return -EINVAL;