dm: core: Add dev_get_uclass_priv() to access uclass private data

Add a convenience function to access the private data that a uclass stores
for each of its devices. Convert over most existing uses for consistency
and to provide an example for others.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_sf.c b/common/cmd_sf.c
index 5c788e9..20f14d3 100644
--- a/common/cmd_sf.c
+++ b/common/cmd_sf.c
@@ -130,7 +130,7 @@
 		return 1;
 	}
 
-	flash = new->uclass_priv;
+	flash = dev_get_uclass_priv(new);
 #else
 	new = spi_flash_probe(bus, cs, speed, mode);
 	if (!new) {