usb: gadget: midi: free hs descriptors

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

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>