coresight: add transferring small packets support for sw usb

Add usb_transfer_small_packet function to support transfer data when
tranferring data size less than USB_BLK_SIZE.

Change-Id: Id4494fb5b649752cdd8550fdf84d9c803897a5f9
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>