flash/nor/core: Fix chunk size calculation in default_flash_mem_blank_check

flash/nor/core: Fix chunk size calculation in default_flash_mem_blank_check

Slow version of blank check procedure reads target memory sector-by-sector
using 1 KB chunks. Due to bug in chunk size calculation algorithm the actual
size of the chunk is always 1 KB even if sector size is smaller.
This causes out-of-boundary read of the last sector.

Steps to reproduce:
1) Use target with small sectors (e.g. psoc6 with 512-byte sectors)
2) set WORKAREASIZE_CM0 0
3) flash erase_check 1

Running slow fallback erase check - add working memory
Info : SWD DPIDR 0x6ba02477
Error: Failed to read memory at 0x14008000
unknown error when checking erase state of flash bank #1 at 0x14000000
	Bank is erased

Change-Id: I03d0d5fb3a1950ae6aac425f5e24c7fd94b38325
Signed-off-by: Bohdan Tymkiv <bhdt@cypress.com>
Reviewed-on: http://openocd.zylin.com/4785
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>