AP_Airspeed: allow zero offset if we are skipping calibration

libraries/AP_Airspeed/AP_Airspeed.h

@@ -123,7 +123,8 @@ public:
     bool healthy(uint8_t i) const {
         bool ok = state[i].healthy && enabled(i);
 #ifndef HAL_BUILD_AP_PERIPH
-        ok &= (fabsf(param[i].offset) > 0 || state[i].use_zero_offset);
+        // sanity check the offset parameter.  Zero is permitted if we are skipping calibration.
+        ok &= (fabsf(param[i].offset) > 0 || state[i].use_zero_offset || param[i].skip_cal);
 #endif
         return ok;
     }