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 <[email protected]>
Reviewed-on: http://openocd.zylin.com/4785
Reviewed-by: Tomas Vanek <[email protected]>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <[email protected]>

Author: Cypress-OpenOCD

src/flash/nor/core.c

@@ -322,8 +322,8 @@ static int default_flash_mem_blank_check(struct flash_bank *bank)
 		for (j = 0; j < bank->sectors[i].size; j += buffer_size) {
 			uint32_t chunk;
 			chunk = buffer_size;
-			if (chunk > (j - bank->sectors[i].size))
-				chunk = (j - bank->sectors[i].size);
+			if (chunk > (bank->sectors[i].size - j))
+				chunk = (bank->sectors[i].size - j);
 
 			retval = target_read_memory(target,
 					bank->base + bank->sectors[i].offset + j,