btuart_read and returned the correct value.

When the nread return value < 0, if the total length ntotal that has been read is > 0, the function returns ntotal; otherwise, it returns the nread error code.

Signed-off-by: zhongzhijie1 <zhongzhijie1@xiaomi.com>

drivers/wireless/bluetooth/bt_uart.c

@@ -86,7 +86,7 @@ static ssize_t btuart_read(FAR struct btuart_upperhalf_s *upper,
       else if (nread < 0)
         {
           wlwarn("Returned error %zd\n", nread);
-          return nread;
+          return ntotal > 0 ? ntotal : nread;
         }
 
       wlinfo("read %zd remaining %zu\n", nread, buflen - nread);