net: ipa: don't pass channel when mapping transaction

Change gsi_channel_trans_map() so it derives the channel used from
the transaction.  Pass the index of the *first* TRE used by the
transaction, and have the called function account for the fact that
the last one used is what's important.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>