tests/bluetooth/ll_settings: Prevent switch-unreachable warning

After the fallthrough comments have been replaced (see commit
741ad17a37807a5396e73c10cf3e5fb425bc87bb) with a macro expanding
to the corresponding attribute, those hints became statements,
and if one happens to occur before any switch case, a warning
reporting it as an unreachable statement is generated.
Prevent the above from happening for function ll_rx_dequeue() in
ull.c by enabling an option that will make some switch case to
appear before this problematic statement.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>