Refactor and unify partner-TXQ handling in the EF100 and legacy drivers.
The main thrust of this series is to remove from the legacy (Siena/EF10)
driver the assumption that a netdev TX queue has precisely two hardware
TXQs (checksummed and unchecksummed) associated with it, so that in
future we can have more (e.g. for handling inner-header checksums) or
fewer (e.g. to free up hardware queues for XDP usage).
Changes from v1:
* better explain patch #1 in the commit message, and rename
xmit_more_available to xmit_pending
* add new patch #2 applying the same approach to ef100, for consistency
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