generic: mxl862xx: allow CPU/SerDes ports to probe on firmware < 1.0.84

generic: mxl862xx: allow CPU/SerDes ports to probe on firmware < 1.0.84

The firmware-version gate in mxl862xx_phylink_get_caps() leaves
config->supported_interfaces empty for the CPU/SerDes ports (9, 13) when
the switch runs firmware older than 1.0.84. phylink rejects an empty
supported_interfaces bitmap, so the switch fails to probe at all:

  mxl862xx mdio-bus:10: switch ready after 2480ms, firmware 1.0.70 (build 70)
  mxl862xx mdio-bus:10: phylink: error: empty supported_interfaces
  error creating PHYLINK: -22
  mxl862xx mdio-bus:10: probe with driver mxl862xx failed with error -22

This regresses boards still shipping the older vendor firmware (e.g. the
BananaPi BPi-R4 Pro 8X, which ships 1.0.70) where all LAN ports disappear.

Ungate only the CPU/SerDes ports (9, 13) so the switch probes and the user
can update the firmware; ports 10..12 and 14..16 stay gated as they are
genuinely unsupported by the old firmware.

Tested on a BPi-R4 Pro 8X with switch firmware 1.0.70 (kernel 6.18); the
6.12 patch receives the identical change for parity.

Fixes: 028dc3f57a6f ("generic: 6.18: update MxL862xx DSA switch driver")
Fixes: 5b69e6a4a658 ("generic: 6.12: update MxL862xx DSA switch driver")
Signed-off-by: Aliaksandr Babrykovich <abobrikovich@gmail.com>
[refreshed patches]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>