msm: ADSPRPC: Clear out sid before freeing buffer

The IOMMU stream ID that the buffer is associated with is
prepended when it gets created to identify the context bank
to be used for the remote processor. This information needs
to be cleared from the buffer address before it is freed.

Change-Id: Iac7af00cd8045a22c0f8e21b3562ecaa720b6cbc
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>