misc: pci_endpoint_test: fix return value of error branch

misc: pci_endpoint_test: fix return value of error branch

We return 'err' in the error branch, but this variable may be set as
zero before. Fix it by setting 'err' as a negative value before we
goto the error label.

Fixes: e03327122e2c ("pci_endpoint_test: Add 2 ioctl commands")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Link: https://lore.kernel.org/r/1605790158-6780-1-git-send-email-wangxiongfeng2@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>