diag: Proper Update of the PD packet buffer status and length

The patch updates the PD packet length prior to
being processed, adds a check for core PD buffer busy
status before being processed and marks the core PD buffers free
only when there is no data to be processed in user PD.

CRs-Fixed: 2068243
Change-Id: Ida215b4df75ddc96ebd5d436850bddf56f3a4fce
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>