net: dsa: b53: Make b53_enable_cpu_port() take a port argument

In preparation for future changes allowing the configuring of multiple
CPU ports, make b53_enable_cpu_port() take a port argument.

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>