AMS: fix usb initialization in bootloader

Regression after 8b8b85433f6e9ac673adb

Change-Id: I86389be0b85c5c2ad8a32d7089a6a79a6b7c8708
diff --git a/bootloader/sansa_as3525.c b/bootloader/sansa_as3525.c
index 14cb2ab..fb3ba98 100644
--- a/bootloader/sansa_as3525.c
+++ b/bootloader/sansa_as3525.c
@@ -137,6 +137,7 @@
     {
 #ifdef HAVE_BOOTLOADER_USB_MODE
         error(EDISK, ret, false);
+        usb_start_monitoring();
         usb_mode();
 #else
         error(EDISK, ret, true);
@@ -152,6 +153,7 @@
     {
 #ifdef HAVE_BOOTLOADER_USB_MODE
         error(EBOOTFILE, ret, false);
+        usb_start_monitoring();
         usb_mode();
 #else
         error(EBOOTFILE, ret, true);