net: dsa: mv88e6xxx: factorize GLOBAL_CONTROL_2 setup

All switch models setup the GLOBAL_CONTROL_2 register with slightly
differences.

Since the cascade mode is valid even in a single chip setup, factorize
such configuration.

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