zephyr: rename remaining CONFIG_USB to CONFIG_USB_DEVICE_STACK

Rename remaining CONFIG_USB to CONFIG_USB_DEVICE_STACK.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index d2f978a..ab3b262 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -186,7 +186,7 @@
 #ifdef CONFIG_SYS_CLOCK_EXISTS
     sys_clock_disable();
 #endif
-#ifdef CONFIG_USB
+#ifdef CONFIG_USB_DEVICE_STACK
     /* Disable the USB to prevent it from firing interrupts */
     usb_disable();
 #endif