misc: pci_endpoint_test: Fix array underflow in pci_endpoint_test_ioctl()

Commit eefb8379 ("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: eefb8379 ("misc: pci_endpoint_test: Add doorbell test case")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/aIzzZ4vc6ZrmM9rI@suswa