staging: line6: add missing MIDI postprocessing case for POD HD300

The driver leaves MIDI processing up to userspace for the POD HD300
device.  Add a missing case statement to skip MIDI postprocessing in the
driver.  This change has no effect other than silencing a printk:

  line6usb driver bug: missing case in linux/drivers/staging/line6/midi.c:179
Signed-off-by: NStefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/line6/midi.c

@@ -173,6 +173,7 @@ static int send_midi_async(struct usb_line6 *line6, unsigned char *data,
 		break;
 
 	case LINE6_DEVID_VARIAX:
+	case LINE6_DEVID_PODHD300:
 		break;
 
 	default: