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.

BACKPORT FROM MAINLINE KERNEL

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: Idf14ca5aded888fe7c56d8b06763b009800827f5