ionic: fix txrx work accounting

Take the tx accounting out of the work_done calculation to
prevent a possible duplicate napi_schedule call when under
high Tx stress but low Rx traffic.

Fixes: b14e4e95 ("ionic: tx separate servicing")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>