net: dsa: mv88e6xx: fix supported_interfaces setup in mv88e6250_phylink_get_caps()

net: dsa: mv88e6xx: fix supported_interfaces setup in mv88e6250_phylink_get_caps()

With the recent PHYLINK changes requiring supported_interfaces to be set,
MV88E6250 family switches like the 88E6020 fail to probe - cmode is
never initialized on these devices, so mv88e6250_phylink_get_caps() does
not set any supported_interfaces flags.

Instead of a cmode, on 88E6250 we have a read-only port mode value that
encodes similar information. There is no reason to bother mapping port
mode to the cmodes of other switch models; instead we introduce a
mv88e6250_setup_supported_interfaces() that is called directly from
mv88e6250_phylink_get_caps().

Fixes: de5c9bf40c45 ("net: phylink: require supported_interfaces to be filled")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20240417103737.166651-1-matthias.schiffer@ew.tq-group.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>