net: dsa: mv88e6xxx: Enable IGMP snooping on user ports only

Do not set the MV88E6XXX_PORT_CTL0_IGMP_MLD_SNOOP bit on CPU or DSA ports.

This allows the host CPU port to be a regular IGMP listener by sending out
IGMP Membership Reports, which would otherwise not be forwarded by the
mv88exxx chip, but directly looped back to the CPU port itself.

Fixes: 54d792f2

 ("net: dsa: Centralise global and port setup code into mv88e6xxx.")
Signed-off-by: Steffen Bätz <steffen@innosonix.de>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230329150140.701559-1-festevam@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>