spi: Add dummy definitions for ACPI lookup functions

Merge series from Richard Fitzgerald <[e-mail]>:

Provide empty versions of acpi_spi_count_resources(),
acpi_spi_device_alloc() and acpi_spi_find_controller_by_adev()
if the real functions are not being built.

This commit fixes two problems with the original definitions:

1) There wasn't an empty version of these functions
2) The #if only depended on CONFIG_ACPI. But the functions are implemented
in the core spi.c so CONFIG_SPI_MASTER must also be enabled for the real
functions to exist.

include/linux/spi/spi.h

···
 					struct spi_controller *ctlr);
 extern void spi_unregister_controller(struct spi_controller *ctlr);
 
-#if IS_ENABLED(CONFIG_ACPI)
+#if IS_ENABLED(CONFIG_ACPI) && IS_ENABLED(CONFIG_SPI_MASTER)
 extern struct spi_controller *acpi_spi_find_controller_by_adev(struct acpi_device *adev);
 extern struct spi_device *acpi_spi_device_alloc(struct spi_controller *ctlr,
 						struct acpi_device *adev,
 						int index);
 int acpi_spi_count_resources(struct acpi_device *adev);
+#else
+static inline struct spi_controller *acpi_spi_find_controller_by_adev(struct acpi_device *adev)
+{
+	return NULL;
+}
+
+static inline struct spi_device *acpi_spi_device_alloc(struct spi_controller *ctlr,
+						       struct acpi_device *adev,
+						       int index)
+{
+	return ERR_PTR(-ENODEV);
+}
+
+static inline int acpi_spi_count_resources(struct acpi_device *adev)
+{
+	return 0;
+}
 #endif
 
 /*