add parens around args in MIDI_EVENT macro

[hexwab]

Yes, this bit me. I had code like the following:

switch (b>>4) {
        case 0x8: case 0x9: case 0xa: case 0xb: case 0xe:
            pkt.Event = MIDI_EVENT(0, b & 0xf0);
            bytes_left = 3; break;
            /* ... */
}