Bluetooth: OBEX: add setpath flags enum

Add documentation for the setpath flags parameter by defining a new
enum bt_obex_setpath_flags that describes the available flags:
- BT_OBEX_SETPATH_FLAG_BACKUP: backup a level before applying name
- BT_OBEX_SETPATH_FLAG_NO_CREATE: don't create folder if it doesn't exist

Update the documentation for bt_obex_setpath() and the setpath callback
in bt_obex_server_ops to reference the new enum, improving clarity for
users of the OBEX setpath operation.

Signed-off-by: Can Wang <can.wang@nxp.com>