<h3>Contribution description</h3>
<p>While testing <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="453403404" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/11658" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/11658/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/11658">#11658</a>, I noticed that setting txmode was broken in the RN2XX3 driver tests application. This PR fixes that.</p>

<h3>Testing procedure</h3>
<ul>
<li>Run <code>tests/driver_rn2xx3</code></li>
<li>Check current txmode and change it to unconfirmable:</li>
</ul>
<pre><code>> mac get txmode
TX mode: confirmable
> mac set txmode uncnf
</code></pre>
<ul>
<li>Verify it's correclty set:</li>
</ul>
<pre><code>> mac get txmode
TX mode: unconfirmable
</code></pre>
<p>On master setting tx mode doesn't work at all, the shell returns the mac command usage.</p>

<h3>Issues/PRs references</h3>
<p>Found while working on <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="453403404" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/11658" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/11658/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/11658">#11658</a></p>


<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/RIOT-OS/RIOT/pull/11659'>https://github.com/RIOT-OS/RIOT/pull/11659</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>tests/driver_rn2xx3: fix invalid element index to txmode</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/11659/files#diff-0">tests/driver_rn2xx3/main.c</a>
    (4)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11659.patch'>https://github.com/RIOT-OS/RIOT/pull/11659.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11659.diff'>https://github.com/RIOT-OS/RIOT/pull/11659.diff</a></li>
</ul>

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