net: ipa: use IDs for last allocated transaction

Use the allocated and free transaction IDs to determine whether the
"last" transaction used for quiescing a channel is in allocated
state.  The last allocated transaction that has not been committed
(if any) immediately precedes the first free transaction in the
transaction array.

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