iio: addac: ad74413r: fix off by one in ad74413r_parse_channel_config()

The > needs to be >= to prevent accessing one element beyond the end of
the st->channel_configs[] array.

Fixes: fea251b6 ("iio: addac: add AD74413R driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>