Fix assert to check flash programming offset

The assert introduced in 00c8648351 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>

src/flash/nor/at91sam3.c

@@ -2845,7 +2845,7 @@ static int sam3_write(struct flash_bank *bank,
 
 	/* By checking that offset is correct here, we also
 	fix a clang warning */
-	assert(offset == pPrivate->page_size);
+	assert(offset % pPrivate->page_size == 0);
 
 	/* intermediate large pages */
 	/* also - the final *terminal* */