iio: add field identifier for @use_count kernel-doc

Kernel-doc for @use_count does not currently have a field identifier.
All the rest of the fields do. @use_count is used internally and should
not be accessed directly by the driver so it should be marked as so.

Add [INTERN] identifier to @use_count field.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>