iio: industrialio-core: iio_write_channel_info accept IIO_VAL_INT_PLUS_NANO

Allow iio_write_channel_info() to accept IIO_VAL_INT_PLUS_NANO

Changes since V1:
use callback to query expected format/precision

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>