ubi: fastmap: Only produce the initial anchor PEB when fastmap is used

Don't produce the initial anchor PEB when ubi device is read-only
or fastmap is disabled, else the resulting PEB will be unusable
to any volume.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>