flash/nor/tcl: Make write_bank parameter optional

Make the 'offset' parameter optional, if omitted simply start at the
beginning of the flash bank.

Additionally, check if the argument is out of bounds of the flash bank.

Change-Id: I8e9632b539ad9e83211e1ac6a06da4c8109cbc60
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3860


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