net: ena: Make missed_tx stat incremental

net: ena: Make missed_tx stat incremental

[ Upstream commit ccd143e5150f24b9ba15145c7221b61dd9e41021 ]

Most statistics in ena driver are incremented, meaning that a stat's
value is a sum of all increases done to it since driver/queue
initialization.

This patch makes all statistics this way, effectively making missed_tx
statistic incremental.
Also added a comment regarding rx_drops and tx_drops to make it
clearer how these counters are calculated.

Fixes: 11095fdb712b ("net: ena: add statistics for missed tx packets")
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>