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

[Nico Huber (Code Review)]

Attention is currently required from: Edward O'Callaghan, Angel Pons, Anastasia Klimchuk.
Nico Huber 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 2:

(2 comments)

File buspirate_spi.c:

https://review.coreboot.org/c/flashrom/+/52958/comment/8328caaa_213fbb26 
PS2, Line 54: bp_
The `bp_` prefix seems unnecessary now.

https://review.coreboot.org/c/flashrom/+/52958/comment/522524d6_f0fd9c98 
PS2, Line 167: 	struct buspirate_spi_data *buspirate_data = data;
How about an

unsigned char *const bp_commbuf = buspirate_data.bp_commbuf;

here and similar cases below? that could reduce some noise, both in code and diff.