stlink: fix max packet size for 8 bit R/W on stlink-v3

stlink: fix max packet size for 8 bit R/W on stlink-v3

While ST internal documentation for STLINK-V3 reports that 8 bits
read/write commands handle 512 bytes of data, a firmware bug makes
it crashing on high data size.
This is fixed with firmware V3J6 (shipped together with V2J36).

Check for firmware version to use the proper data size.

Change-Id: Iaba6cd26bbe130097c1c19de610680e0e8b69bfc
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: https://sourceforge.net/p/openocd/tickets/259/
Reviewed-on: http://openocd.zylin.com/5408
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>