HAL_ChibiOS: always send zero DShot when disarmed

this prevents a misconfigured system (for example SERVOn_REVERSED=1)
from running a motor while disarmed.

See https://discuss.ardupilot.org/t/plane-4-1-0-beta/72434/34?u=tridge

libraries/AP_HAL_ChibiOS/RCOutput.cpp

@@ -1334,6 +1334,7 @@ void RCOutput::dshot_send(pwm_group &group, uint32_t time_out_us)
     }
 #endif
     bool safety_on = hal.util->safety_switch_state() == AP_HAL::Util::SAFETY_DISARMED;
+    bool armed = hal.util->get_soft_armed();
 
     memset((uint8_t *)group.dma_buffer, 0, DSHOT_BUFFER_LENGTH);
 
@@ -1383,6 +1384,11 @@ void RCOutput::dshot_send(pwm_group &group, uint32_t time_out_us)
                 value += 47;
             }
 
+            if (!armed) {
+                // when disarmed we always send a zero value
+                value = 0;
+            }
+
             // according to sskaug requesting telemetry while trying to arm may interfere with the good frame calc
             bool request_telemetry = telem_request_mask & chan_mask;
             uint16_t packet = create_dshot_packet(value, request_telemetry, group.bdshot.enabled);