net: dsa: add bridge member iteration macro

net: dsa: add bridge member iteration macro

Drivers that offload bridges need to iterate over the ports that are
members of a given bridge, for example to rebuild per-port forwarding
bitmaps when membership changes. Currently drivers typically open-code
this by combining dsa_switch_for_each_user_port() with a
dsa_port_offloads_bridge_dev() check, or cache bridge membership
within the driver.

Add dsa_switch_for_each_bridge_member() macro to express this pattern
directly, and use it for the existing dsa_bridge_ports() inline
helper.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/e7136aaa26773f39e805a00fe4ecf13cd2b83fc0.1775049897.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>