iio:iio_buffer_init(): Only set watermark if not already set

Only initialize the watermark field if it is still 0. This allows drivers
to provide a custom default watermark value. E.g. some driver might have a
fixed watermark or can only support watermarks within a certain range and
the initial value for the watermark should be within this range.

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