Change in flashrom[master]: spi_master: Use new API to register shutdown function

[Nico Huber (Code Review)]

Attention is currently required from: Thomas Heijligen, Angel Pons, Anastasia Klimchuk.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/56103 )

Change subject: spi_master: Use new API to register shutdown function
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Patch Set 1:

(1 comment)

Patchset:

PS1:
> I have a message to reviewers, thank you!
> I don't know how to build ni845x_spi

This is Miklós' arcane knowledge :)
> raiden_debug_spi is left for now, it has different data used in register_spi_master vs register_shutdown

Oh, this one looks funny. The data free'd in the shutdown function is
the master struct itself :) So what we would actually need is a
field in `data` pointing back to the master. Currently it's free'd
like this (where `spi_master` is currently called `spi_config` in the
code):

free(spi_master->data)
    free(spi_master)

If we add a pointer to `data`, pointing to the master, we could turn
it around:

free(data->master)
    free(data)

And then we could use the same `data` pointer everwhere.