mlx5en: Statically allocate and free the channel structure(s).

By allocating the worst case size channel structure array
at attach time we can eliminate various NULL checks in the
fast path. And also reduce the chance for use-after-free
issues in the transmit fast path.

This change is also a requirement for implementing
backpressure support.

Submitted by: hselasky@
Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies