ft2232: warn about lack of TRST on "turtle" adapter

ft2232: warn about lack of TRST on "turtle" adapter

This patch fixes a build error (assigning a variable to itself) with
clang.

Since this adapter lacks trst, trying to use it in reset_config will
fail silently. Warn the user accordingly.

Change-Id: I16fd20936f00d7ff82962f4fcc629ff434aa4dce
Reported-by: Rainer Müller <raimue@codingfarm.de>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1946
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>