net: dsa: sja1105: Make dynamic_config_read return -ENOENT if not found

Conceptually, if an entry is not found in the requested hardware table,
it is not an invalid request - so change the error returned
appropriately.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>