powerpc/powernv: Fix major bugs in opal_flash

• The BIO bio_data may not be page aligned. Only the base address of each page worth of data is extracted to pass to OPAL. Without page alignment it can scribble over random memory when finishing the page read. Fix this by short-reading the first page to properly align for full page reads.
• Fix the definition of OPAL_FLASH_ERASE.
• Properly handle the async message result, as now returned from r345974.