ena: Count all currently missing TX completions in check

Currently we count all of the newly added and already existing
missing tx completions in each iteration of
check_missing_comp_in_tx_queue() causing duplicate counts
to missing_tx_comp stat.

This commit adds a new counter new_missed_tx within the relevant
function which only counts the newly added missing tx completions
in each iteration of check_missing_comp_in_tx_queue().
This will allow us to update missing_tx_comp stat accurately without
counting duplicates.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.