soc/amd/stoneyridge: Improve code quality

Remove empty functions. In function pointer structure
"device_operations", replace the 0 equality by NULL equality.
Files: hda.c, sata.c and usb.c

Change-Id: I9f8dc7681ab2e651872e69a8b2e990e59ebe80c9
Signed-off-by: Richard Spiegel <richard.spiegel@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/21522
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/soc/amd/stoneyridge/usb.c b/src/soc/amd/stoneyridge/usb.c
index c1caf7d..82984b9 100644
--- a/src/soc/amd/stoneyridge/usb.c
+++ b/src/soc/amd/stoneyridge/usb.c
@@ -26,16 +26,12 @@
 	.set_subsystem = pci_dev_set_subsystem,
 };
 
-static void usb_init(struct device *dev)
-{
-}
-
 static struct device_operations usb_ops = {
 	.read_resources = pci_ehci_read_resources,
 	.set_resources = pci_dev_set_resources,
 	.enable_resources = pci_dev_enable_resources,
-	.init = usb_init,
-	.scan_bus = 0,
+	.init = NULL,
+	.scan_bus = NULL,
 	.ops_pci = &lops_pci,
 };