net: dsa: constify cpu_dp member of dsa_port

A DSA port has a dedicated CPU port assigned to it, stored in the cpu_dp
member. It is not meant to be modified by a port, thus make it const.

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