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 3: Code-Review+1

(3 comments)

File buspirate_spi.c:

https://review.coreboot.org/c/flashrom/+/52958/comment/cd75ce9e_7512a654 
PS3, Line 58: unsigned char **bp_commbuf
> I was considering the idea to pass a pointer to `struct bp_spi_data` into buspirate_commbuf_grow but […]

Well, it's roughly the same either way. Local variables and bp_data need to be
synchronized after a change. So after a call to buspirate_commbuf_grow(), it's
either

bp_commbuf = bp_data->bp_commbuf;

or

bp_data->bp_commbuf = bp_commbuf;

As `bp_commbufsize` is only consumed in buspirate_commbuf_grow() we could even
save us a local variable for it when passing `bp_data`. We'd have to switch
allocation of `bp_commbuf` and `bp_data`, though.

This can all change later, however. This patch looks good already.

https://review.coreboot.org/c/flashrom/+/52958/comment/88dacd1f_0f3a1441 
PS3, Line 329: 	unsigned char *bp_commbuf = NULL;
             : 	int bp_commbufsize = 0;
             : 
Nit, `NULL`/`0` should never be consumed, so the initializations are unnecessary.

https://review.coreboot.org/c/flashrom/+/52958/comment/e44066da_ca50916c 
PS3, Line 695: 	/* Storing the latest values into data, commbuf migth have grown during init. */
             : 	bp_data->bp_commbuf = bp_commbuf;
             : 	bp_data->bp_commbufsize = bp_commbufsize;
In the light of the discussion above, this shouldn't be needed?