flash/nor/stm32lx: use 0 base to autodetect second bank location

Change-Id: I3c296b3e276fcd4d92e4180fc0d2133eebfcc240
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2503


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