misc: pci_endpoint_test: Avoid triggering a BUG()

If you call ida_simple_remove(&pci_endpoint_test_ida, id) with a negative
"id" then it triggers an immediate BUG_ON().  Let's not allow that.

Fixes: 2c156ac7 ("misc: Add host side PCI driver for PCI test function device")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>