iio: buffer: Use IIO_SEPARATE instead of a hard-coded 0

iio: buffer: Use IIO_SEPARATE instead of a hard-coded 0

Use an explicit IIO_SEPARATE instead of 0 for the 'shared_by' parameter
when calling __iio_add_chan_devattr().

For some reason, commit 3704432fb1fd ("iio: refactor info mask and ext_info
attribute creation.") updated only 1 place out of 4.
Update the remaining ones now.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/1d17f57423172fcb9d9797cfe7c8282f356049c2.1702831285.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>