stm32f2x: Correct calculation of number of 128k sectors.

Fixes regression introduced in 9785f51f that caused write outside
allocated memory during probe of devices with <1024K flash.

Reported-by: Alexander Pakhomov <ker0sin@yandex.ru>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Change-Id: Ifba8833e436064624efbf678162538fd351a5702
Reviewed-on: http://openocd.zylin.com/1450
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>