In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/iio/accel/mma8452.c: In function ‘mma8452_probe’:
drivers/iio/accel/mma8452.c:1581:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ret == data->chip_info->chip_id)
      ^
drivers/iio/accel/mma8452.c:1584:2: note: here
  default:
  ^~~~~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>