usb: gadget: f_midi: drop substreams when disabling endpoint

This change makes sure that the ALSA buffers are cleaned if an endpoint
becomes disabled.

Before this change, if the internal ALSA buffer did overflow, the MIDI
function would stop sending MIDI to the host.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>