bgpd: Fix `no set as-path prepend ASNUM...`

bgpd: Fix `no set as-path prepend ASNUM...`

If entering `no set as-path prepend 1 2 3`, it's warned as unknown command.

Now fixed, and the following combinations work fine:

```
no set as-path prepend
no set as-path prepend last-as
no set as-path prepend last-as 1
no set as-path prepend 1
no set as-path prepend 1 2
```

Fixes: https://github.com/FRRouting/frr/issues/15912
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit e069a1c8d52bb6e7808303df488d7ce912e4f652)