iio: __iio_update_buffers: Perform request_update() only for new buffers

We only have to call the request_update() callback for a newly inserted
buffer. The configuration of the already previously active buffers will not
have changed.

This also allows us to move the request_update() call to the beginning of
__iio_update_buffers(), before any currently active buffers are stopped.
This makes the error handling a lot easier since no changes were made to
the buffer list and no rollback needs to be performed.

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