stlink: add support for 16 bit memory read/write

stlink: add support for 16 bit memory read/write

Recent stlink version V2J26 introduces support for 16 bit memory
read/write.
Add the new API and modify the wrappers stlink_usb_{read|write}_mem
For older version of stlink keep the same behavior as in current
code.
While there, fix some minor typo in comments inside the wrappers.
This fixes ticket #204 ("st-link can't flash stm32f2x based flash
without target algorithms").

Change-Id: Id7d404e588f10a4b0f8a93d7ca44cef8e4c49b4f
Author: Laurent LEMELE <laurent.lemele@st.com>
Rebased-by: Antonio Borneo <borneo.antonio@gmail.com>
Ticket: https://sourceforge.net/p/openocd/tickets/204/
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4704
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>