dm: spi: Correct BIOS protection logic for ICH9

The logic is incorrect and currently has no effect. Fix it so that we can
write to SPI flash, since by default it is write-protected.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Andrew Bradford <andrew.bradford@kodakalaris.com>