Sansa AMS: fix usb in bootloader

No need to define HAVE_BOOTLOADER_USB_MODE
That define might need some cleaning up
Fix FS#12384

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@31241 a1c6a512-1295-4272-9138-f99709370657
diff --git a/bootloader/sansa_as3525.c b/bootloader/sansa_as3525.c
index c384007..7b0a8b8 100644
--- a/bootloader/sansa_as3525.c
+++ b/bootloader/sansa_as3525.c
@@ -63,8 +63,10 @@
                 (LCD_HEIGHT - SYSFONT_HEIGHT) / 2, msg);
     lcd_update();
 
+    usb_enable(true);
     while(usb_detect() == USB_INSERTED)
         sleep(HZ);
+    usb_enable(false);
 
     reset_screen();
     lcd_update();