iio: refactor info mask and ext_info attribute creation.

Introduce an enum to specify whether the attribute is separate or
shared.

Factor out the bitmap handling for loop into a separate function.

Tidy up error handling and add a NULL assignment to squish a false
positive warning from GCC.

Change ext_info shared type from boolean to enum and update in all
drivers.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 96e97ad..7ea2edb 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -208,7 +208,7 @@
 				     &iio_show_scan_index,
 				     NULL,
 				     0,
-				     0,
+				     IIO_SEPARATE,
 				     &indio_dev->dev,
 				     &buffer->scan_el_dev_attr_list);
 	if (ret)