msm: adsprpc: Remove output buffers from cache flush logic

Flush only input buffers during cache maintenance, removing output
buffers from flush logic, helpful when target or buffer is
non-coherent.

Acked-by: Deepika Singh <dsi@qti.qualcomm.com>
Change-Id: If84c5bcc0a136d2943ab96c137d0561b8ef8d47d
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>