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

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.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43891

(cherry picked from commit e05436d577de98944b97b9cf510b29c4d2091b3f)