samples: modules: lvgl: fix integer overflow

`lv_task_handler()` returns a `uint32_t`, but `k_msleep` takes a
`int32_t`.

If no timer exists, `lv_task_handler()` returns `UINT32_MAX` to indicate
that we should wait forever. However, this gets auto-cast to `-1`,
indicating to `k_msleep` to not wait at all, creating a busy loop.

Hence, a clamping to `[0, INT32_MAX]` is required.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>