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: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>