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>

drivers/acpi/internal.h

@@ -67,6 +67,11 @@ struct acpi_ec {
 
 extern struct acpi_ec *first_ec;
 
+#ifdef	CONFIG_ACPI_PCI_SLOT
+void acpi_pci_slot_init(void);
+#else
+static inline void acpi_pci_slot_init(void) { }
+#endif
 int acpi_pci_root_init(void);
 int acpi_ec_init(void);
 int acpi_ec_ecdt_probe(void);