When a feed has submodules, all its submodules are fully cloned whereas
the feed itself is shallowed. Let's be consistent and perform shallow clones
as well for the submodules.
Signed-off-by: Cedric CHEDALEUX <cedric.chedaleux@orange.com>
Link: https://github.com/openwrt/openwrt/pull/18003
Signed-off-by: Robert Marko <robimarko@gmail.com>