Documentation: PCI: endpoint: Use the correct return value of pcitest.sh

The pci_endpoint_test driver has been fixed to return correct error no
from IOCTL. In that process, the pcitest tool now returns SUCCESS and
FAILED instead of OKAY and NOT_OKAY. So change that in documentation also.

Cc: stable@vger.kernel.org #5.10
Fixes: 16263d9e ("Documentation: PCI: Add userguide for PCI endpoint test function")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>