Fix USB speed detection

Change-Id: I615d2d5366ffa1e2e9c6b0837bf5a62a7e471fca
Reviewed-on: http://gerrit.rockbox.org/994
Reviewed-by: Frank Gevaerts <frank@gevaerts.be>
Tested: Mihail Zenkov <mihail.zenkov@gmail.com>
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>
diff --git a/bootloader/sansa_as3525.c b/bootloader/sansa_as3525.c
index 01fb845..3558d01 100644
--- a/bootloader/sansa_as3525.c
+++ b/bootloader/sansa_as3525.c
@@ -124,11 +124,13 @@
 
 #ifdef USE_ROCKBOX_USB
     usb_init();
-    usb_start_monitoring();
 
     /* Enter USB mode if USB is plugged and SELECT button is pressed */
-    if(btn & BUTTON_SELECT && usb_detect() == USB_INSERTED)
-        usb_mode();
+    if(btn & BUTTON_SELECT) {
+        usb_start_monitoring();
+        if(usb_detect() == USB_INSERTED)
+            usb_mode();
+    }
 #endif /* USE_ROCKBOX_USB */
 
     while((ret = disk_mount_all()) <= 0)