add parens around args in MIDI_EVENT macro

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;
    			/* ... */
    }