msm: ADSPRPC: Buffer length truncated while validation

The buffer length that is being used to validate gets truncated
due to it being assigned to wrong type causing invalid memory
to be accessed when the actual buffer length is used to copy
user buffer contents.

Change-Id: If04dee27b8bae04eef7455773d9f4327fd008a21
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>