net: dsa: lantiq_gswip: introduce bitmap for MII ports

net: dsa: lantiq_gswip: introduce bitmap for MII ports

Instead of relying on hard-coded numbers for MII ports, introduce
a bitmap for MII ports.
This is done in order to prepare for supporting MaxLinear GSW1xx ICs
which got a different layout of ports.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/019fc8ed06f2317976eac143320d1dc046e8f392.1755878232.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>