hsu: workaround premature TX DMA irq

hsu: workaround premature TX DMA irq

BZ: 25843

When CTS assert, HSU TX DMA will stop and report TX DMA irq, which is a
violation against the HSU spec.

Workaround this by reading D0SAR and count the real chars sent -- don't
assume all TX chars are sent.

Change-Id: Ic43f40e1d2437d6e33f8d53b84d116168f921a45
Signed-off-by: Alek Du <alek.du@intel.com>
Reviewed-on: http://android.intel.com:8080/49696
Reviewed-by: Yang, Bin <bin.yang@intel.com>
Tested-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>