mlx5en: Add more error checks in the transmit path.

• Upon error more completion events than requested may be generated, particularly when using the completion event factor feature.
• Count number of event errors in the transmit path.

Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit 4f4739a77b0e69dae57fd1687926d6e48a698fe4)