netlink: specs: remove implicit structs for SNMP counters

uAPI doesn't define structs for the SNMP counters, just enums to index
them as arrays. Switch to the same representation in the spec. C codegen
will soon need all the struct types to actually exist.

Note that the existing definition was broken, anyway, as the first
member should be the number of counters reported.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250506194101.696272-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>