net: dsa: use cpu_dp in master code

Make it clear that the master device is linked to a CPU port by using
"cpu_dp" for the dsa_port variable in master.c instead of "port", then
use a "port" variable to describe the port index, as usually seen in
other places of DSA core.

This will make the future patch touching dsa_ptr more readable. There is
no functional changes.

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>