ft2232: fix input scan ending in drshift/irshift

The final bit was incorrectly added as output data, even if no data was
to be written. Changed it to match handling of other bits.

Change-Id: I91e5ba0c932876bfb579c22e6c7ef0300baa1534
Signed-off-by: Yann Vernier <yann.vernier@orsoc.se>
Reviewed-on: http://openocd.zylin.com/1049
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>