sfc: Fix transposed ptp_{under, over}size_sync_windows statistics

Somehow I transposed these two while bringing the original statistics
support upstream.

Fixes: 99691c4a ('sfc: Add PTP counters to ethtool stats')
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>