usb: Drop use of BUG_ON() and WARN_ON()

These macros use __FILE__ which inserts the full path of the object file
into U-Boot, thus increasing file size. Drop these usages.

An older version of this patch was submitted here:

http://patchwork.ozlabs.org/patch/1205784/

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/gadget/f_mass_storage.c b/drivers/usb/gadget/f_mass_storage.c
index 45c7b58..c1e6506 100644
--- a/drivers/usb/gadget/f_mass_storage.c
+++ b/drivers/usb/gadget/f_mass_storage.c
@@ -390,7 +390,11 @@
 	if (common->fsg)
 		return 1;
 	ERROR(common, "common->fsg is NULL in %s at %u\n", func, line);
+#ifdef __UBOOT__
+	assert_noisy(false);
+#else
 	WARN_ON(1);
+#endif
 	return 0;
 }