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

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

Change-Id: I8f4c1b161c103a77bdb30c6bf052293b5ed48c41
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3861


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>