openrisc: Fix segv jsp due to free of unallocated data

openrisc: Fix segv jsp due to free of unallocated data

The prompt pointer in the jtag serial port is never zero'd or allocated.
Completely remove it since there is not much use for it as the target
software will provide the actual prompt.

Change-Id: Id95d8ccb9f725e53b9d03386b11d91eba1cd6ef4
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-on: http://openocd.zylin.com/4093
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>