net: ipa: track completed transactions with an ID

Add a transaction ID field to track the first element in the
transaction array that has completed but has not yet been polled.

Advance the ID when we are processing a transaction in the NAPI
polling loop (where completed transactions become polled).

Temporarily add warnings that verify that the first completed
transaction tracked by the ID matches the first element on the
completed list, both when pending and completing.

Remove the temporary warnings added by the previous commit.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>