mutual-tls: fix clang-tidy warning

```
../http/mutual_tls.hpp:77:35: error: unsafe buffer access [-Werror,-Wunsafe-buffer-usage]
        unsigned char usageChar = usage->data[i];
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Icc6905f31fdd54b683fe7807eb72e9b78437b2d1