arm: allwinner: aw_spi: Call bus_generic_attach

This is needed when the driver is compiled into the kernel.
When compiled as a module this will be called from another
code path as we also depend on ofw_spibus.

MFC after:	1 week

sys/arm/allwinner/aw_spi.c

@@ -242,7 +242,7 @@ aw_spi_attach(device_t dev)
 
 	sc->spibus = device_add_child(dev, "spibus", -1);
 
-	return (0);
+	return (bus_generic_attach(dev));
 
 fail:
 	aw_spi_detach(dev);