ACPI / PCI: Make pci_slot built-in only, not a module

As discussed in thread at https://patchwork.kernel.org/patch/1946851/,
there's no value in supporting CONFIG_ACPI_PCI_SLOT=m any more.
So change Kconfig and code to only support building pci_slot as
built-in driver.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
index 7c43bdc..236e476 100644
--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c
@@ -1687,6 +1687,7 @@
 
 	acpi_power_init();
 	acpi_pci_root_init();
+	acpi_pci_slot_init();
 
 	/*
 	 * Enumerate devices in the ACPI namespace.