net: systemport: Fix 64-bit statistics dependency

There are several problems with commit 10377ba7 ("net: systemport:
Support 64bit statistics", first one got fixed in 7095c973 ("net:
systemport: Fix 64-bit stats deadlock").

The second problem is that this specific code updates the
stats64.tx_{packets,bytes} from ndo_get_stats64() and that is what we
are returning to ethtool -S. If we are not running a tool that involves
calling ndo_get_stats64(), then we won't get updated ethtool stats.

The solution to this is to update the stats from both call sites,
factoring that into a specific function, While at it, don't just check
the sizeof() but also the type of the statistics in order to use the
64-bit stats seqlock.

Fixes: 10377ba7 ("net: systemport: Support 64bit statistics")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>