net: dsa: lantiq_gswip: Use dev_err_probe where appropriate

dev_err_probe() can be used to simplify the existing code. Also it means
we get rid of the following warning which is seen whenever the PMAC
(Ethernet controller which connects to GSWIP's CPU port) has not been
probed yet:
  gswip 1e108000.switch: dsa switch register failed: -517

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20240611135434.3180973-5-ms@dev.tdt.de


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