net: ip: utils: revise data type declarations in `z_impl_net_addr_ntop`

Revise data type declarations to avoid implicit/explicit casts:
- Changed `unsigned char` to `uint8_t` for `zeros` array
  to match with `longest`.
- Declared variables `i`, `j`, and `pos` as `int` to use the same type.
- Cast `value` to `uint16_t` instead of `uint32_t` since
  `value` is `uint16_t`.
- Moved `bh` and `bl` declarations to narrow the scope of variables.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>