net: dsa: felix: suppress -EPROBE_DEFER errors

The DSA master might not have been probed yet in which case the probe of
the felix switch fails with -EPROBE_DEFER:
[    4.435305] mscc_felix 0000:00:00.5: Failed to register DSA switch: -517

It is not an error. Use dev_err_probe() to demote this particular error
to a debug message.

Fixes: 56051948

 ("net: dsa: ocelot: add driver for Felix switch family")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220408101521.281886-1-michael@walle.cc
Signed-off-by: Jakub Kicinski <kuba@kernel.org>