usb: Introduce CONFIG_SPL_DM_USB

This allows building the SPL without driver model for USB. Since
CONFIG_SPL_DM_USB is enabled if and only if CONFIG_DM_USB was enabled
before, this patch does not change the build behaviour.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>
diff --git a/drivers/usb/host/Makefile b/drivers/usb/host/Makefile
index cb8c315..bfa565c 100644
--- a/drivers/usb/host/Makefile
+++ b/drivers/usb/host/Makefile
@@ -3,7 +3,7 @@
 # (C) Copyright 2000-2007
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-ifdef CONFIG_DM_USB
+ifdef CONFIG_$(SPL_)DM_USB
 obj-$(CONFIG_CMD_USB) += usb-uclass.o
 obj-$(CONFIG_SANDBOX) += usb-sandbox.o
 endif