iio: fix config watermark initial value

config structure is set to 0 when updating the buffers, so by
default config->watermark will be 0. When computing the minimum
between config->watermark and the buffer->watermark or
insert_buffer-watermark, this will always be 0 regardless of the
value set by the user for the buffer.

Set as initial value for config->watermark the maximum allowed
value so that the minimum value will always be set from one of the
buffers.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Fixes: f0566c0c405d ("iio: Set device watermark based on watermark of all
attached buffers")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-buffer.c

@@ -653,6 +653,7 @@ static int iio_verify_update(struct iio_dev *indio_dev,
 	unsigned int modes;
 
 	memset(config, 0, sizeof(*config));
+	config->watermark = ~0;
 
 	/*
 	 * If there is just one buffer and we are removing it there is nothing