flash/nor/tcl.c: fix flash bank bounds check in 'flash fill' command handler

flash/nor/tcl.c: fix flash bank bounds check in 'flash fill' command handler

Steps to reproduce ( STM32F103 'Blue Pill', 128KiB of flash ):
> flash fillh 0x0801FFFE 00 1
wrote 2 bytes to 0x0801fffe in 0.019088s (0.102 KiB/s)
> flash fillw 0x0801FFFE 00 1
Error: stm32f1x.cpu -- clearing lockup after double fault
Error: error waiting for target flash write algorithm
Error: error writing to flash at address 0x08000000 at offset 0x0001fffe

Change-Id: I145092ec5e45bc586b3df48bf37c38c9226915c1
Signed-off-by: Bohdan Tymkiv <bhdt@cypress.com>
Reviewed-on: http://openocd.zylin.com/4516
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>