hw/block/onenand: Remove dead code block

The condition of the for-loop makes sure that b is always smaller
than s->blocks, so the "if (b >= s->blocks)" statement is completely
superfluous here.

Buglink: https://bugs.launchpad.net/qemu/+bug/1715007
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
