iio: __iio_update_buffers: Split enable and disable path into helper functions

__iio_update_buffers is already a rather large function with many different
error paths and it is going to get even larger. This patch factors out the
device enable and device disable paths into separate helper functions.

The patch also re-implements iio_disable_all_buffers() using the new
iio_disable_buffers() function removing a fair bit of redundant code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>