!53 更改 "rt_device_read()" 和"rt_device_write()" 的注释错误

Merge pull request !53 from Jackistang/fix

src/device.c

@@ -307,7 +307,7 @@ RTM_EXPORT(rt_device_close);
  *
  * @param size is the size of buffer.
  *
- * @return the actually read size on successful, otherwise negative returned.
+ * @return the actually read size on successful, otherwise zero returned.
  *
  * @note since 0.4.0, the unit of size/pos is a block for block device.
  */
@@ -350,7 +350,7 @@ RTM_EXPORT(rt_device_read);
  *
  * @param size is the size of buffer.
  *
- * @return the actually written size on successful, otherwise negative returned.
+ * @return the actually written size on successful, otherwise zero returned.
  *
  * @note since 0.4.0, the unit of size/pos is a block for block device.
  */