staging:iio: Disallow modifying buffer size when buffer is enabled

staging:iio: Disallow modifying buffer size when buffer is enabled

The buffer buffer storage is only update when enabling the buffer. Changing the
buffer size while the buffer is enabled will confuse the buffer in regard to
its actual buffer size and can cause potential memory corruption. Thus it is
only safe to modify the buffer size when the buffer is disabled.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>