netlink: specs: rt-link: remove implicit structs from devconf

netlink: specs: rt-link: remove implicit structs from devconf

devconf is even odder than SNMP. On input it reports an array of u32s
which seem to be indexed by the enum values - 1. On output kernel
expects a nest where each attr has the enum type as the nla type.

sub-type: u32 is probably best we can do right now.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250506194101.696272-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>