iio: core: Add and export __iio_dev_mode_lock()

iio: core: Add and export __iio_dev_mode_lock()

Add unconditional wrappers around the internal IIO mode lock.

As mentioned in the documentation, this is not meant to be used by
drivers, instead this will aid in the eventual addition of cleanup
classes around conditional locks.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Kurt Borja <kuurtb@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>