net: macb: Clean up the .usrio settings in macb_config instances

net: macb: Clean up the .usrio settings in macb_config instances

All instances of macb_config currently have the .usrio set, but most of
them use &macb_default_usrio. In fact, there is no need to duplicate
this across all macb_config instances. Remove the .usrio setting from
instances that use &macb_default_usrio, and ensure that the default is
selected at runtime when no other value is explicitly set.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Link: https://patch.msgid.link/20260310-macb-cleanup-v1-3-928c1a91a7dc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>