misc: pci_endpoint_test: Fix array underflow in pci_endpoint_test_ioctl()

Commit eefb83790a0d ("misc: pci_endpoint_test: Add doorbell test case")
added NO_BAR (-1) to the pci_barno enum which, in practical terms,
changes the enum from an unsigned int to a signed int. If the user
passes a negative number in pci_endpoint_test_ioctl() then it results in
an array underflow in pci_endpoint_test_bar().

Fixes: eefb83790a0d ("misc: pci_endpoint_test: Add doorbell test case")
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Manivannan Sadhasivam <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Link: https://patch.msgid.link/aIzzZ4vc6ZrmM9rI@suswa

drivers/misc/pci_endpoint_test.c

···
 	switch (cmd) {
 	case PCITEST_BAR:
 		bar = arg;
-		if (bar > BAR_5)
+		if (bar <= NO_BAR || bar > BAR_5)
 			goto ret;
 		if (is_am654_pci_dev(pdev) && bar == BAR_0)
 			goto ret;