net: dsa: mv88e6xxx: don't set non-existing learn2all bit for 6220/6250

net: dsa: mv88e6xxx: don't set non-existing learn2all bit for 6220/6250

The 6220 and 6250 switches do not have a learn2all bit in global1, ATU
control register; bit 3 is reserverd.

On the switches that do have that bit, it is used to control whether
learning frames are sent out the ports that have the message_port bit
set. So rather than adding yet another chip method, use the existence
of the ->port_setup_message_port method as a proxy for determining
whether the learn2all bit exists (and should be set).

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Link: https://lore.kernel.org/r/20201210110645.27765-1-rasmus.villemoes@prevas.dk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>