msm: adsprpc: Fix virtual address for already mapped buffers

Virtual address was being set incorrectly for already mapped buffers 
which was leading to invalid access in DSP. Fix is to set proper
virtual address.

Change-Id: I51407f20bb44f8fa1dd1f4c1951db3c34d153252
Acked-by: Tadakamalla Krishnaiah <ktadakam@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>