misc: pci_endpoint_test: Fix the return value of IOCTL

IOCTLs are supposed to return 0 for success and negative error codes for
failure. Currently, this driver is returning 0 for failure and 1 for
success, that's not correct. Hence, fix it!

Cc: stable@vger.kernel.org #5.10
Fixes: 2c156ac7 ("misc: Add host side PCI driver for PCI test function device")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>