DMA with zero-length transfers doesn't make sense and this HW doesn't
support them at all, so increase the threshold.

Fixes: fc66b39f ("i2c: mediatek: Use DMA safe buffers for i2c transactions")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
[wsa: reworded commit message]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>