hw/block: m25p80: Don't write to flash if write is disabled

hw/block: m25p80: Don't write to flash if write is disabled

When write is disabled, the write to flash should be avoided
in flash_write8().

Fixes: 82a2499011a7 ("m25p80: Initial implementation of SPI flash device")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-id: 1608688825-81519-1-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>