nfp: expose ring stats of inactive rings via ethtool

After user changes the ring count statistics for deactivated
rings disappear from ethtool -S output.  This causes loss of
information to the user and means that ethtool stats may not
add up to interface stats.  Always expose counters from all
the rings.  Note that we allocate at most num_possible_cpus()
rings so number of rings should be reasonable.

The alternative of only listing stats for rings which were
ever in use could be confusing.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>