implement stlink_usb_send and use it to fix stlink_usb_write_mem
using two calls to stlink_usb_recv is inappropriate since each
call issues a SG command on stlink-v1, resulting in errors
Change-Id: I24ef9f2dda284e041dc4a532b59968a77eebe702
Signed-off-by: Szymon Modzelewski <szmodzelewski@gmail.com>
Reviewed-on: http://openocd.zylin.com/498
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>