net: dsa: b53: Initialize ds->ops in b53_switch_alloc

In order to allow drivers to override specific dsa_switch_driver
callbacks, initialize ds->ops to b53_switch_ops earlier, which avoids
having to expose this structure to glue drivers.

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