jlink: Replace crazy home cooked assertion with a real one

Change-Id: I83d66ddef34965dd94a0ff76aa1367cc9d19f2df
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1644


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>