msm: ADSPRPC: Skip file mapping for static shell

Do not map buffer if the file length is passed as
zero from user space for static shell linking use cases.

Change-Id: I992f22e12e88d2f019dfb646987ec7eaeb5ab40c
Acked-by: Himateja Reddy <hmreddy@qti.qualcomm.com>
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>