ASoC: rt5514: reset dma_offset at hw_params

dma_offset needs reset every time otherwise for consecutive PCM
open the last dma_offset value will cause incorrect pointer position
be reported to user space.

Signed-off-by: Hsin-Yu Chao <hychao@chromium.org>
Signed-off-by: Oder Chiou <oder_chiou@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>