net: dsa: mv88e6xxx: factorize GLOBAL_MONITOR_CONTROL setup

All switch drivers configure the GLOBAL_MONITOR_CONTROL register with
slightly changes.

Assume the setup of the upstream port, and configure it as the port to
which ingress and egress and ARP monitor frames are to be sent.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>