drivers/at: make at_dev_init() return uart initialize status

in case there's an error uart_init() returns an error value,
this value indicates the reason for the error thus we should
return it from at_dev_init() so the user will be able to identify
whether the init succeded or failed.

drivers/at/at.c

@@ -31,10 +31,9 @@ int at_dev_init(at_dev_t *dev, uart_t uart, uint32_t baudrate, char *buf, size_t
 {
     dev->uart = uart;
     isrpipe_init(&dev->isrpipe, buf, bufsize);
-    uart_init(uart, baudrate, _isrpipe_write_one_wrapper,
-              &dev->isrpipe);
 
-    return 0;
+    return uart_init(uart, baudrate, _isrpipe_write_one_wrapper,
+                     &dev->isrpipe);
 }
 
 int at_expect_bytes(at_dev_t *dev, const char *bytes, uint32_t timeout)

drivers/include/at.h

@@ -124,8 +124,8 @@ typedef struct {
  * @param[in]   buf         input buffer
  * @param[in]   bufsize     size of @p buf
  *
- * @returns     0 on success
- * @returns     <0 otherwise
+ * @returns     success code UART_OK on success
+ * @returns     error code UART_NODEV or UART_NOBAUD otherwise
  */
 int at_dev_init(at_dev_t *dev, uart_t uart, uint32_t baudrate, char *buf, size_t bufsize);
 

tests/driver_at/main.c

@@ -47,9 +47,19 @@ static int init(int argc, char **argv)
     uint8_t uart = atoi(argv[1]);
     uint32_t baudrate = atoi(argv[2]);
 
-    at_dev_init(&at_dev, UART_DEV(uart), baudrate, buf, sizeof(buf));
+    int res = at_dev_init(&at_dev, UART_DEV(uart), baudrate, buf, sizeof(buf));
 
-    return 0;
+    /* check the UART initialization return value and respond as needed */
+    if (res == UART_NODEV) {
+        puts("Invalid UART device given!");
+        return 1;
+    }
+    else if (res == UART_NOBAUD) {
+        puts("Baudrate is not applicable!");
+        return 1;
+    }
+
+    return res;
 }
 
 static int send(int argc, char **argv)