msm: ADSPRPC: Buffer length to be copied is truncated

The buffer length that is being used to allocate gets truncated
due to it being assigned to wrong type causing a much smaller
buffer to be allocated than what is required for copying.

Change-Id: I30818acd42bd282837c7c7aa16d56d3b95d4dfe7
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>