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.dkSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -1347,9 +1347,16 @@ static int mv88e6xxx_atu_setup(struct mv88e6xxx_chip *chip)
 	if (err)
 		return err;
 
+	/* The chips that have a "learn2all" bit in Global1, ATU
+	 * Control are precisely those whose port registers have a
+	 * Message Port bit in Port Control 1 and hence implement
+	 * ->port_setup_message_port.
+	 */
+	if (chip->info->ops->port_setup_message_port) {
 		err = mv88e6xxx_g1_atu_set_learn2all(chip, true);
 		if (err)
 			return err;
+	}
 
 	return mv88e6xxx_g1_atu_set_age_time(chip, 300000);
 }