[thci] correct conversion of IPv6 prefix (#3846)
[thci] correct conversion of IPv6 prefix (#3846)


librasungirl/openthread,openthread/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,turon/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,jwhui/openthread,chshu/openthread,librasungirl/openthread,openthread/openthread,bukepo/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,librasungirl/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,turon/openthread,lanyuwen/openthread,librasungirl/openthread,turon/openthread,bukepo/openthread,abtink/openthread,srickardti/openthread,abtink/openthread,turon/openthread,chshu/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,openthread/openthread