docs: fix info about representor identification

Update the "How are representors identified?" documentation
subchapter. For newer kernels driver should use
SET_NETDEV_DEVLINK_PORT instead of ndo_get_devlink_port()
callback.

Fixes: 7712b3e9

 ("Merge branch 'net-fix-netdev-to-devlink_port-linkage-and-expose-to-user'")
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/20231012123144.15768-1-mateusz.polchlopek@intel.com


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