Bluetooth: Mesh: fix different signedness

CC:  subsys/bluetooth/mesh/prov.c
subsys/bluetooth/mesh/prov.c: In function 'bt_mesh_input_string':
subsys/bluetooth/mesh/prov.c:616:14:
        warning: pointer targets in passing argument 1 of
        'strncpy' differ in signedness [-Wpointer-sign]
  616 |  strncpy(link.auth, str, prov->input_size);
      |          ~~~~^~~~~
      |              |
      |              uint8_t * {aka unsigned char *}

Signed-off-by: chao an <anchao@xiaomi.com>