msm: ADSPRPC: Fix buffer length for capability API

Buffer size was being set incorrectly. Buffer length for dsp attributes
was being set to 4 times its actual buffer size. This was causing
stack-out-of-bounds when trying to copy back the buffer on function
put_args.
Fix is to set buffer length to the correct buffer size of dsp attributes.

Change-Id: I755034ca503d58443fa6f14bf03c18d448b290b4
Signed-off-by: Edgar Flores <edgarf@codeaurora.org>