Change in flashrom[master]: buspirate_spi.c: Refactor singleton states into reentrant pattern

[Anastasia Klimchuk (Code Review)]

Attention is currently required from: Nico Huber, Edward O'Callaghan, Angel Pons.
Anastasia Klimchuk has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/52958 )

Change subject: buspirate_spi.c: Refactor singleton states into reentrant pattern
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Patch Set 3:

(2 comments)

File buspirate_spi.c:

https://review.coreboot.org/c/flashrom/+/52958/comment/bf5dbd9e_23229ca3 
PS2, Line 54: bp_
> The `bp_` prefix seems unnecessary now.

Yes sure, I usually do this as separate patch - so now there is CB:54886 on the top of the chain.

https://review.coreboot.org/c/flashrom/+/52958/comment/428cd28f_28fc3f97 
PS2, Line 167: 	struct buspirate_spi_data *buspirate_data = data;
> Thank you so much for detailed explanation, and I am really sorry for delay from my side! During las […]

After some more thinking, I did this:
I really wanted to keep as much optimisations from the Author as possible, but also reduce noise and diffs because that was the suggestion which started this thread.
So bp_data is still spi_master data (keeps optimisations), but I use a shortcut local variable in most of functions (reduce noise and diffs).