net: dsa: mxl-gsw1xx: validate chip ID

net: dsa: mxl-gsw1xx: validate chip ID

No check for actually present hardware is being performed in the probe
function of the mxl-gsw1xx switch driver. So even if the switch isn't
present at the configured MDIO bus address the driver wrongly tells the
user that a "GSWIP version 0 mod 0" was found, outputting errors about
PHY capabilities not matching.

Read and validate the chip MANU_ID and PNUM_ID registers and output
information while probing, but return an error and abort probing in case
the hardware is not actually present.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/3194d3d3bb0b51f08755d392e1fdf7bb6dc49608.1769916962.git.daniel@makrotopia.org
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>