sfxge: Move txq->next pointer to part writable on completion path

[arybchik]

Summary

In fact the pointer is used only if more than one TXQ is processed in
one
interrupt.
It is used (read-write) on completion path only.
Also it makes the first part of the structure smaller and it fits now
into one 128byte cache line. So, TXQ structure becomes 128 bytes
smaller.

Sponsored by: Solarflare Communications, Inc.

[gnn]

Remember to put Approved by: gnn (mentor) in the commit message.

[arybchik]

Committed to head