USB: gadget: don't wait for completion twice

In some obscure scenarios e.g. passing a 0-byte backing file
storage, wait_for_completion() would wait forever in fsg_cleanup().

Prevent it by completing the thread in fsg_bind() error path.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>