Bluetooth: L2CAP: Fix checking if LTK is present

This fix a typo where incorrect member of bt_keys was used for
checking if LTK is present. This was resulting in bogus results
depending on connection role and current identity used.

This was affecting L2CAP/LE/CFC/BV-25-C qualification test case.

Fixes: #42862

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>