Add a transaction ID field to track the first element in a channel's
transaction array that has been committed, but not yet passed to the
hardware.  Advance the ID when the hardware is notified via doorbell
that TREs from a transaction are ready for consumption.

Temporarily add warnings that verify that the first committed
transaction tracked by the ID matches the first element on the
committed list, both when committing and pending (at doorbell).

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>