Add additional doorbells on ENA Tx path

The new ENA HAL is introducing API, which can determine on Tx path if
the doorbell is needed.

That way, it can tell the driver, that it should call an doorbell.
The old threshold value wasn't removed, as not all HW is supporting this
feature - so it was reworked to also work with the new API.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.