net: dsa: sja1105: Shim declaration of struct sja1105_dyn_cmd

This structure is merely an implementation detail and should be hidden
from the sja1105_dynamic_config.h header, which provides to the rest of
the driver an abstract access to the dynamic configuration interface of
the switch.

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