net: dsa: bcm_sf2: Fix driver removal

With the DSA core doing the call to dsa_port_disable() we do not need to
do that within the driver itself. This could cause an use after free
since past dsa_unregister_switch() we should not be accessing any
dsa_switch internal structures.

Fixes: 0394a63a

 ("net: dsa: enable and disable all ports")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>