Change in flashrom[master]: ft2232_spi.c: Add ft2232_spi_send_multicommand()

[Simon Buhrow (Code Review)]

Simon Buhrow has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/40477 )

Change subject: ft2232_spi.c: Add ft2232_spi_send_multicommand()
......................................................................

Patch Set 19:
> So, the correct way to do this is to implement `.multicommand`
> instead of `.command`. I would roughly do it as follows:
> Loop
>     Put write part of command into buffer.
>     If not last command and read part is empty,
>         Continue with next loop iteration.
>     Execute buffer.
>     Execute read part if any.
> While not last command.

I uploaded PS18 with my first draft of ft2232_spi_send_multicommand().

Having Patch 40748 in mind, I decided to pack max one read cmd into one ftdi call.
This simplifies the readback pointer management as well (readarr).
Furthermore I think the majority (all?) calls of ft2232_spi_send_multicommand() will have at most one read cmd (correct me if I´m wrong). 
(To be honest I´m not sure if ft2232_spi_send_multicommand() is ever called with read cmd in the current implementation)

I´m not sure if the loop condition is safe. It´s copied from default_spi_send_multicommand().

It´s untested!! I´ll do this the next week(s).
Comments are welcome.