net: dsa: bcm_sf2: Utilize b53_{enable, disable}_port

Export b53_{enable,disable}_port and use these two functions in
bcm_sf2_port_setup and bcm_sf2_port_disable. The generic functions
cannot be used without wrapping because we need to manage additional
switch integration details (PHY, Broadcom tag etc.).

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>