qcom-sps-dma: fix a memory leak and improve performance

Rather than allocating an async descriptor per user's
scatter-gather list, a single async descriptor is now a
member of each channel. This new descriptor points to the
user's scatter-gather list. Thus avoids allocation of
an alternative container and copying the content to it.
As a result the memory leak is fixed and performance is
improved.

Change-Id: Ibb7c7233384db661b604815977be10ba71c6f5e0
Signed-off-by: Ankit Gupta <ankgupta@codeaurora.org>