iio: Set device watermark based on watermark of all attached buffers

Currently the watermark of the device is only set based on the watermark
that is set for the user space buffer. This doesn't consider the watermarks
set on any attached in-kernel buffers.

Change this so that the watermark of the device should be the minimum of
the watermarks over all attached buffers.

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