Set the ACK retry delay for radio communication

def set_ard_time(self, us):
        """ Set the ACK retry delay for radio communication """
        # Auto Retransmit Delay:
        # 0000 - Wait 250uS
        # 0001 - Wait 500uS
        # 0010 - Wait 750uS
        # ........
        # 1111 - Wait 4000uS

        # Round down, to value representing a multiple of 250uS
        t = int((us / 250) - 1)
        if (t < 0):
            t = 0
        if (t > 0xF):
            t = 0xF
        _send_vendor_setup(self.handle, SET_RADIO_ARD, t, 0, ())