msm: ADSPRPC: Avoid multiple hypervisor assign calls for a buffer

Access to dma shared buffer is set at device probe, avoid making
multiple hypervisor assign calls for the same buffer.

Change-Id: I91f7dd0bca109fa774af49159bdec57b8acd65b2
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>
Acked-by: Viswanatham Paduchuri <vpaduchu@qti.qualcomm.com>