slim_ngd: Handle invalid/timeout TX errors gracefully

Invalid transactions can cause timeout or NACK TX errors.
Clear completion object reference for pending TX messages and print
TX BAM pipe descriptors and status registers for such transactions.
TX timeouts during connect/disconnect port error conditions should
clear completion object reference for acklowledgement receipt
to avoid referring to stale completion object later.

Change-Id: I81018df8cab59b43753c46c8a49a5066b794fda6
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>