eth: fbnic: wrap tx queue stats in a struct

The queue stats struct is used for Rx and Tx queues. Wrap
the Tx stats in a struct and a union, so that we can reuse
the same space for Rx stats on Rx queues.

This also makes it easy to add an assert to the stat handling
code to catch new stats not being aggregated on shutdown.

Acked-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20250211181356.580800-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>