net: dsa: make cpu_dp non const

A port may trigger operations on its dedicated CPU port, so using
cpu_dp as const will raise warnings. Make cpu_dp non const.

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