cfi_flash: Add missing check for erased dest to flash_write_cfibuffer()

cfi_flash: Add missing check for erased dest to flash_write_cfibuffer()

The check for an sufficiently erased destination was missing in the
buffered write function of the cfi flash driver (when
CFG_FLASH_USE_BUFFER_WRITE is defined). This patch adds this check to that
writing to such a region will fail with the currect error message.

Signed-off-by: Stefan Roese <sr@denx.de>