net: dsa: bcm_sf2: Declare our own dsa_switch_ops

Utilize the b53 exported functions to fill our bcm_sf2_ops structure,
also making it clear what we utilize and what we specifically override.

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