usb: qcserial avoid pointing to freed memory

Rework the qcprobe logic such that serial->private is not set when
qcprobe exits with -ENODEV, otherwise serial->private will point to freed
memory on -ENODEV

Signed-off-by: Steven Hardy <shardy@redhat.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>