net: dsa: mv88e6xxx: move trunk setup

Move the trunking setup out of Global 2 specific setup into the top
level mv88e6xxx_setup function.

Note that the 88E6390 family calls this LAG instead of Trunk and
supports 32 possible ID routing vectors, with LAG ID bit 4 being placed
in Global 2 register 0x1D...

We don't need Trunk (or LAG) IDs for the moment, thus keep it simple.

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