platform/surface: aggregator: Allow is_ssam_device() to be used when CONFIG_SURFACE_AGGREGATOR_BUS is disabled

In SSAM subsystem drivers that handle both ACPI and SSAM-native client
devices, we may want to check whether we have a SSAM (native) client
device. Further, we may want to do this even when instantiation thereof
cannot happen due to CONFIG_SURFACE_AGGREGATOR_BUS=n. Currently, doing
so causes an error due to an undefined reference error due to
ssam_device_type being placed in the bus source unit.

Therefore, if CONFIG_SURFACE_AGGREGATOR_BUS is not defined, simply let
is_ssam_device() return false to prevent this error.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20220527023447.2460025-2-luzmaximilian@gmail.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

include/linux/surface_aggregator/device.h

@@ -177,6 +177,8 @@ struct ssam_device_driver {
 	void (*remove)(struct ssam_device *sdev);
 };
 
+#ifdef CONFIG_SURFACE_AGGREGATOR_BUS
+
 extern struct bus_type ssam_bus_type;
 extern const struct device_type ssam_device_type;
 
@@ -193,6 +195,15 @@ static inline bool is_ssam_device(struct device *d)
 	return d->type == &ssam_device_type;
 }
 
+#else /* CONFIG_SURFACE_AGGREGATOR_BUS */
+
+static inline bool is_ssam_device(struct device *d)
+{
+	return false;
+}
+
+#endif /* CONFIG_SURFACE_AGGREGATOR_BUS */
+
 /**
  * to_ssam_device() - Casts the given device to a SSAM client device.
  * @d: The device to cast.