Bluetooth: Mesh: fix different signedness
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 <218d4e837dd886ea0ff93702c797a8007ccb76b5@xiaomi.com>

galak/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr