msm: ADSPRPC: Initialize FastRPC invoke metadata

Initializing metadata buffer to zero before reusing the buffer
for next invoke.

Change-Id: Iaab3478732b83427a475e95afa0e031cb76f60d9
Acked-by: Viswanatham Paduchuri <vpaduchu@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>