stlink-v1: fix memory writes

stlink-v1: fix memory writes

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: I52ff9ee8f5d9ae0d47356477468eb98952205c99
Signed-off-by: Szymon Modzelewski <szmodzelewski@gmail.com>
Reviewed-on: http://openocd.zylin.com/478
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>