mac80211: consider per-CPU statistics if present

If we have been keeping per-CPU statistics, consider them
regardless of USES_RSS, because we may not actually fill
those, for example in non-fast-RX cases when the connection
is not compatible with fast-RX. If we didn't fill them, the
additional data will be zero and not affect anything, and
if we did fill them then it's more correct to consider them.

This fixes an issue in mesh mode where some statistics are
not updated due to USES_RSS being set, but fast-RX isn't
used.

Reported-by: Thiraviyam Mariyappan <tmariyap@codeaurora.org>
Link: https://lore.kernel.org/r/20210610220814.13b35f5797c5.I511e9b33c5694e0d6cef4b6ae755c873d7c22124@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>