[thci] fix warning when set downgrade threshold for ncp device (#3113)
[thci] fix warning when set downgrade threshold for ncp device (#3113)


LeZhang2016/openthread,jwhui/openthread,librasungirl/openthread,jwhui/openthread,turon/openthread,gandreello/openthread,chshu/openthread,lanyuwen/openthread,erja-gp/openthread,erja-gp/openthread,srickardti/openthread,srickardti/openthread,lanyuwen/openthread,LeZhang2016/openthread,srickardti/openthread,openthread/openthread,gandreello/openthread,bukepo/openthread,gandreello/openthread,erja-gp/openthread,abtink/openthread,jwhui/openthread,erja-gp/openthread,erja-gp/openthread,abtink/openthread,jwhui/openthread,LeZhang2016/openthread,chshu/openthread,erja-gp/openthread,openthread/openthread,chshu/openthread,librasungirl/openthread,turon/openthread,gandreello/openthread,LeZhang2016/openthread,turon/openthread,chshu/openthread,gandreello/openthread,lanyuwen/openthread,LeZhang2016/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,turon/openthread,gandreello/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,openthread/openthread,bukepo/openthread,bukepo/openthread,lanyuwen/openthread,LeZhang2016/openthread,abtink/openthread,librasungirl/openthread,abtink/openthread