drivers: ipc_router: Reset and free partial pkt during SSR

After subsystem restart, SMD XPRT is reading the new packet data
such as HELLO packet into the partial packets read before SSR. This
causes to miss the important control packets and stall the IPC router
communication.

Check and free the partial packet during the XPRT close in SSR case.

CRs-Fixed: 893827
Change-Id: Ifc91b7ccc568da05893d8eaab10f44148cdc3f6d
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>