net: ipa: simplify TX completion statistics

When a TX request is issued, its channel's accumulated byte and
transaction counts are recorded.  This currently does *not* take
into account the transaction being committed.

Later, when the transaction completes, the number of bytes and
transactions that have completed since the transaction was committed
are reported to the network stack.  The transaction and its byte
count are accounted for at that time.

Instead, record the transaction and its bytes in the counts recorded
at commit time.  This avoids the need to do so when the transaction
completes, and provides a (small) simplification of that code.

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