usb: gadget: f_midi: fix error recovery path

In case kstrdup() fails the resources to release are
midi->in_port[] and midi. No cards have been registered,
so no need to unregister any.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>