net: dsa: felix: fix ethtool 256-511 and 512-1023 TX packet counters

What the driver actually reports as 256-511 is in fact 512-1023, and the
TX packets in the 256-511 bucket are not reported. Fix that.

Fixes: 56051948

 ("net: dsa: ocelot: add driver for Felix switch family")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>