net/ena: assert on outstanding mbuf in Tx

net/ena: assert on outstanding mbuf in Tx

To make sure there is no outstanding mbuf in the reused Tx queue (due to
improper cleanup, or some invalid logic on Tx path), the assertion was
added on the Tx path.

As it's being compiled out in the release version, it won't affect
the IO path performance.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Dawid Gorecki <dgr@semihalf.com>
Reviewed-by: Shai Brandes <shaibran@amazon.com>