Fix assert to check flash programming offset

Fix assert to check flash programming offset

The assert introduced in 00c864835149a96b431fc8f31dd89542d88fd383 checks
whether the programming offset equals to page_size of the flash, while it
wants to check whether the offset is a multiple of the page_size.

Change-Id: I794d021951a28c1cc520b5eea5d500f097721b06
Signed-off-by: Attila Kinali <attila@kinali.ch>
Reviewed-on: http://openocd.zylin.com/482
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>