usb: gadget: midi: free hs descriptors

commit d185039f7982eb82cf8d03b6fb6689587ca5af24 upstream.

The HS descriptors are only created if HS is supported by the UDC but we
never free them.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>