net: ipa: stop counting total RX bytes and transactions

In gsi_evt_ring_rx_update(), we update each transaction so its len
field reflects the actual number of bytes received.  In the process,
the total number of transactions and bytes processed on the channel
are summed, and added to a running total for the channel.

But we don't actually use those running totals for RX endpoints.
They're maintained for TX channels to support CoDel when they are
associated with a "real" network device.

So stop maintaining these totals for RX endpoints, and update the
comment where the fields are defined to make it clear they're only
valid for TX channels.

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