coresight: fix unsigned integer calculation overflow

The cumulative value of small_size may be greater than USB_BLK_SIZE
in usb_transfer_small_packe, the result of "usb_blk_size-small_size"
overflows in usb_read_work_fn. This change fix this issue.

Change-Id: I8719c206592893743efc83b1ceac6c117bd7a24a
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>