update ds18b20.c source code, add break when find ds18b20

Guo Wenxue

2018-10-21 6ad80326dd46f2e557367f7845f91ed1949a07e9

 apue/1.File_IO/ds18b20.c

@@ -76,6 +76,7 @@
            /* find and get the chipset SN filename */
            strcpy(chip,direntp->d_name);
            found = 1;
       break;
        }
    }
    closedir(dirp);