msm: sps: add flexible memory allocation for desc cache

When a descriptor FIFO is large, the cache for descriptors also requires
relatively large memory allocation. Use vmalloc for this case. For the
small memory allocation, still use kmalloc.

Change-Id: Ibf40dd7b7917ed6472a9fcfd3fb77ecdcf505926
Signed-off-by: Yan He <yanhe@codeaurora.org>