net: ethtool: fix __ethtool_dev_mm_supported() implementation

net: ethtool: fix __ethtool_dev_mm_supported() implementation

The MAC Merge layer is supported when ops->get_mm() returns 0.
The implementation was changed during review, and in this process, a bug
was introduced.

Link: https://lore.kernel.org/netdev/20230111161706.1465242-5-vladimir.oltean@nxp.com/
Fixes: 04692c9020b7 ("net: ethtool: netlink: retrieve stats from multiple sources (eMAC, pMAC)")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ferenc Fejes <fejes@inf.elte.hu>
Link: https://lore.kernel.org/all/20230220122343.1156614-2-vladimir.oltean@nxp.com/
Signed-off-by: Jakub Kicinski <kuba@kernel.org>