static const unsigned char wheel_tbl[2][4] =
    {
       /* 0x00  0x40  0x80  0xc0 */ /* Wheel value        */
        { 0x40, 0xc0, 0x00, 0x80 }, /* Clockwise rotation */
        { 0x80, 0x00, 0xc0, 0x40 }, /* Counter-clockwise  */
    };

...
        if (old_wheel_value == wheel_tbl[0][wheel_value >> 6])
            btn = BUTTON_SCROLL_DOWN;
        else if (old_wheel_value == wheel_tbl[1][wheel_value >> 6])
            btn = BUTTON_SCROLL_UP;
...
    old_wheel_value = wheel_value;