boot-lantiq: Fix compilation with GCC 14

boot-lantiq: Fix compilation with GCC 14

This fixes the following compile problem:
```
arv7506pw11.c: In function 'show_boot_progress':
arv7506pw11.c:59:24: error: 'return' with a value, in function returning void [-Wreturn-mismatch]
   59 |                 return 0;
      |                        ^
arv7506pw11.c:56:6: note: declared here
   56 | void show_boot_progress(int arg)
      |      ^~~~~~~~~~~~~~~~~~
arv7506pw11.c:71:16: error: 'return' with a value, in function returning void [-Wreturn-mismatch]
   71 |         return 0;
      |                ^
arv7506pw11.c:56:6: note: declared here
   56 | void show_boot_progress(int arg)
      |      ^~~~~~~~~~~~~~~~~~
```

Link: https://github.com/openwrt/openwrt/pull/18688
Signed-off-by: Hauke Mehrtens <[email protected]>