net: dsa: Encapsulate legacy switch drivers into dsa_switch_driver

In preparation for making struct dsa_switch_ops const, encapsulate it
within a dsa_switch_driver which has a list pointer and a pointer to
dsa_switch_ops. This allows us to take the list_head pointer out of
dsa_switch_ops, which is written to by {un,}register_switch_driver.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>