bgpd: Use enum bgp_create_error_code as argument in header

bgpd: Use enum bgp_create_error_code as argument in header

```
bgpd/bgp_vty.c:865:5: warning: conflicting types for ‘bgp_vty_return’ due to enum/integer mismatch; have ‘int(struct vty *, enum bgp_create_error_code)’ [-Wenum-int-mismatch]
  865 | int bgp_vty_return(struct vty *vty, enum bgp_create_error_code ret)
      |     ^~~~~~~~~~~~~~
In file included from ./bgpd/bgp_mplsvpn.h:15,
                 from bgpd/bgp_vty.c:48:
./bgpd/bgp_vty.h:148:12: note: previous declaration of ‘bgp_vty_return’ with type ‘int(struct vty *, int)’
  148 | extern int bgp_vty_return(struct vty *vty, int ret);
      |            ^~~~~~~~~~~~~~
```

Fixing stuff regarding GCC13.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>