net: dsa: realtek-smi: don't log an error on EPROBE_DEFER

Probe deferral is not an error, so don't log this as an error:

[0.590156] realtek-smi ethernet-switch: unable to register switch ret = -517

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>