tools: PCI: Fix parsing the return value of IOCTLs

"pci_endpoint_test" driver now returns 0 for success and negative error
code for failure. So adapt to the change by reporting FAILURE if the
return value is < 0, and SUCCESS otherwise.

Cc: stable@vger.kernel.org #5.10
Fixes: 3f2ed813 ("tools: PCI: Add a userspace tool to test PCI endpoint")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>