msm: ipa: use dma_pool for sps_transfer buffer

dma_alloc_coherent does minimum of page sized allocations,
invoking this from atomic context increases the odds of a
failed allocation which hurts data path performance. The
size of allocation is fixed and quite small compared to
page size. Use a dma_pool which uses DMA memory optimally.

Change-Id: I742bb97e23073a367a157988dba3efbc75cfb833
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>