net: dsa: mt7530: get cpu-port via dp->cpu_dp instead of constant

Replace last occurences of hardcoded cpu-port by cpu_dp member of
dsa_port struct.

Now the constant can be dropped.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>