subsys: storage: flash_map: Fix warning

Prior to this commit, `flash_area_layout()` was being passed a pointer
to the incorrect type (`uint32_t *` where `int *` was expected).  This
caused the following warning to be reported:

```
[...]/subsys/storage/flash_map/flash_map.c: In
function 'flash_area_get_sectors':
[...]/subsys/storage/flash_map/flash_map.c:191:32:
warning: passing argument 2 of 'flash_area_layout' from incompatible
pointer type [-Wincompatible-pointer-types]
  return flash_area_layout(idx, cnt, ret, get_sectors_cb, &data);
                                ^~~
[...]/subsys/storage/flash_map/flash_map.c:136:12:
note: expected 'int *' but argument is of type 'uint32_t * {aka long
unsigned int *}'
 static int flash_area_layout(int idx, int *cnt, void *ret,
            ^~~~~~~~~~~~~~~~~
```

This commit changes the argument type to `u32_t` for both functions.

Signed-off-by: Christopher Collins <ccollins@apache.org>