Refactor variable names and comments for clarity in Firmware.asm

[david-schmidt]

I poked at a few insignificant things while working on my understanding of the driver:

• "latch" was hard to grok for me as not-a-hardware person; specify when we tickle hardware I/O
• Comment some more as the code progresses due to my expanding understanding
• Capacity as reported by status I think was wrong by half
• Restore zero page using Y-index loop similar to the way they were saved, just backwards