net: ipa: rework gsi_channel_tx_update()

Rename gsi_channel_tx_update() to be gsi_trans_tx_completed(), and
pass it just the transaction pointer, deriving the channel from the
transaction.  Update the comments above the function to provide a
more concise description of how statistics for TX endpoints are
maintained and used.

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