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>