Bluetooth: Controller: llcp: Set remote LLCP rx_opcode to type unused

Some of control procedures has set ctx->rx_opcode in rp_comm_tx
function to values that state there is an expected PDU.
This is not always true. In case of procedures that do not expect
any response from remote device, the ctx->rx_opcode should be set
to PDU_DATA_LLCTRL_TYPE_UNUSED.

In the worst cases scenario, when the same control procedure
is executed locally and remotely, correct response PDU may
cause an assertion in  ull_cp_rx. It could happen because
of wrong ctx->rx_opcode value. A packet with opcode that is
set in remote and local control procedure context rx_opcode
will be treated as expected value for both. That is a situation
that cannot happen.

The commit changes the assignments of rx_opcode to fix the problem.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>