treewide: Test on CONFIG_USB_HOST (or CONFIG_CMD_USB) not CONFIG_USB

As the logic here is only used when we have a USB host controller, test
on CONFIG_USB_HOST rather than CONFIG_USB in general.  This lets us move
towards using CONFIG_USB only as a menu symbol to say that we have some
form of USB, and then USB_HOST or USB_GADGET depending on the role that
USB plays within the build.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/tplink/wdr4300/wdr4300.c b/board/tplink/wdr4300/wdr4300.c
index 9134d6b..f2b9210 100644
--- a/board/tplink/wdr4300/wdr4300.c
+++ b/board/tplink/wdr4300/wdr4300.c
@@ -15,7 +15,7 @@
 #include <mach/ddr.h>
 #include <debug_uart.h>
 
-#ifdef CONFIG_USB
+#ifdef CONFIG_USB_HOST
 static void wdr4300_usb_start(void)
 {
 	void __iomem *gpio_regs = map_physmem(AR71XX_GPIO_BASE,