eth: fbnic: add support for basic qstats

eth: fbnic: add support for basic qstats

Implement netdev_stat_ops and export the basic per-queue stats.

This interface expect users to set the values that are used
either to zero or to some other preserved value (they are 0xff by
default). So here we export bytes/packets/drops from tx and rx_stats
plus set some of the values that are exposed by queue stats
to zero.

  $ cd tools/testing/selftests/drivers/net && ./stats.py
  [...]
  Totals: pass:4 fail:0 xfail:0 xpass:0 skip:0 error:0

Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20240810054322.2766421-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>