soc: qcom: ipc_router: Fix memory leak issue

Fix memory leak issue by calling kfree() after successful
tx of packet over MHI.
Add lock while freeing resources in ul path.

Change-Id: I027e3ab331643ae88782f3ad8f516a16a11dbecd
Signed-off-by: Arun Prakash <app@codeaurora.org>