i2c: add extra check to safe DMA buffer helper

Make sure we report 'no buffer' for 0-length messages. This can only
happen if threshold is set to 0 which is kind of bogus but we should
still handle this situation. Update the docs and add a debug message
to educate callers of this function.

Reported-by: Hsin-Yi Wang <hsinyi@chromium.org>
Fixes: e94bc5d1 ("i2c: add helpers to ease DMA handling")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>