lpc2000: ignore status of part ID IAP command

The IAP firmware won't return a proper status with some versions. This
happens on my CCC r0ket board and others have seen it as well [1]. So
just ignore the status code and do a (weak) consistency check instead.

[1] http://www.lpcware.com/content/forum/lpc1343-iap-read-part-identification-command

Change-Id: I0daa779d520a540629677c56857bbc20d6db422d
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Reviewed-on: http://openocd.zylin.com/2364
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>