acpi: Don't assume a resource is reserved in acpi_delete_resource

[jhb]

Summary

This fixes a panic if a driver uses bus_set_resource to add a resource
that fails to reserve and then deletes the resource via
bus_delete_resource.

[imp]

I'm surprised this hasn't bitten sooner

[jhb]

In D43891#1001335, @imp wrote:

I'm surprised this hasn't bitten sooner

Very few things use bus_delete_resource. I only hit this when I ran into the ram0 problem and my changes to pci_host_generic caused a bridge to fail to probe when it couldn't allocate its window,