Bluetooth: host: df: Fix wrong HCI cmd buf size for RX params set

In case the direction finding RX is enabled but without Angle
of Arrival functionality enabled, there HCI command buffer
had wrong command length assigned. It didn't take into account
two dummy antenna IDs provided to fulfil requirement from
BT 5.3 Core Vol 4, Part E sections: 7.7.82, 7.7.85.

HCI driver when received such command buffer, didn't handle it
due to wrong command length.

The commit fixes the problem.

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