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

Rather then 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 and thus avoids allocation of an alternative container and
copying the content to it. As a result a memory leak is fixed and
performance is also improved.

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