net: dsa: bcm_sf2: Do not clobber b53_switch_ops

We make the bcm_sf2 driver override ds->ops which points to
b53_switch_ops since b53_switch_alloc() did the assignent. This is all
well and good until a second b53 switch comes in, and ends up using the
bcm_sf2 operations. Make a proper local copy, substitute the ds->ops
pointer and then override the operations.

Fixes: f458995b ("net: dsa: bcm_sf2: Utilize core B53 driver when possible")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>