hw/nand: reject read-only drives

also gracefully fail on nand_device_init() for unsupported block
size instead of aborting.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>