Move back off timer to target struct

Move the global target back off timer to the target struct. This will
fix the wrong error handling with multi target devices like smp systems.

Change-Id: Ia327182ed5d13ca87323700017a8c40ecc6b25a3
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/1179
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>