iio: iio_format_list() should set stride=1 for IIO_VAL_CHAR

iio_format_list() sets a stride across the val array of 1 for INT
type, and 2 for all others. Add IIO_VAL_CHAR so it also gets a
stride of 1 assuming val is an array of integers with char type
values.

No drivers currently use this, but I found this issue adding an
avail callback for IIO_INFO_THERMOCOUPLE_TYPE for a driver I'm
working on.

Signed-off-by: Ben Collins <bcollins@kernel.org>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/vidvwybkm3vwmtopihyaj6tlvswwa5ixmgptfzpk5ujl2ixjjb@olz6275ftabs
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>