tools: ynl-gen: prepare for submsg structs

tools: ynl-gen: prepare for submsg structs

Prepare for constructing Struct() instances which represent
sub-messages rather than nested attributes.
Restructure the code / indentation to more easily insert
a case where nested reference comes from annotation other
than the 'nested-attributes' property. Make sure we don't
construct the Struct() object from scratch in multiple
places as the constructor will soon have more arguments.

This should cause no functional change.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250515231650.1325372-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>