flash/nor/tcl: Respect flash bank boundary in verify_bank

Respect the flash bank boundary and compare only the remaining content
of the bank even if the file content is larger.

Change-Id: I4d75979c7893fdd4d18372fa6b0321a0486b4fa9
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3859


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>