net: macb: Fix offset error in gem_update_stats

hw_stats now has only one variable for tx_octets/rx_octets, so we should
only increment p once, not twice. This would cause the statistics to be
reported under the wrong categories in `ethtool -S --all-groups` (which
uses hw_stats) but not `ethtool -S` (which uses ethtool_stats).

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Fixes: f6af690a ("net: cadence: macb: Report standard stats")
Link: https://patch.msgid.link/20250825172134.681861-1-sean.anderson@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>