usb: gadget: f_midi: fix leak on failed to enqueue out requests

This patch fixes a memory leak that occurs when an endpoint fails to enqueue
the request. If that happens the complete function will never be called, thus
never freeing the request.

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