net: ipa: use IDs exclusively for last transaction

Always use transaction IDs when finding the "last" transaction to
await when quiescing a channel.  This basically extends what was
done in the previous patch to all other transaction state IDs.

As a result we are no longer updating any transaction lists inside
gsi_channel_trans_last(), so there's no need to take the spinlock.

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