Bluetooth: tester: Fixes for GCC 10.x

Fix the structures so that they are compliant with the stricter checks
of GCC 10.x, and at the same time correct a bug that was present in the
handling of the GAP Start Advertising BTP command.

Fixes #28375.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>