efc: Avoid repeated afe_adc_spis in Shuttler IO

[atseml]

Having repeated afe_adc_spi entries for disparate Shuttler AFE HW revs caused the ADC SPI communication of Shuttler AFE v1.3 to be incoherent, and it seemed to behave the same regardless of the specified Shuttler AFE HW rev.

[sbourdeauducq]

I see another bug: the AFE I/O is appended to the global list every time EfcShuttler is instantiated, so the global list accumulates them.

artiq/artiq/gateware/targets/efc.py

Line 277 in 4aec72d

shuttler_io.append(afe_adc_io)

I suggest:
(1) capitalizing the names of global variables to clearly mark them as such
(2) either:

shuttler_io = SHUTTLER_IO + afe_adc_io

or, simply:

platform.add_extension(afe_adc_io)

This bug is silent right now since the class is only instantiated once per Python process, but it's certainly not best practices and could cause confusion in future use cases.

[atseml]

(2) either:

shuttler_io = SHUTTLER_IO + afe_adc_io

or, simply:

platform.add_extension(afe_adc_io)

These will require wrapping afe_adc_io in a list to work. I went one step further though and just defined shuttler_io_afe_pre_v1_3 and shuttler_io_afe_v1_3 (deduplicated of course with shuttler_io -> shuttler_io_common) since I believe that's cleaner.

For (1) all Python global constants will be capitalized in a separate PR since that's going to be a repo-wide change.

[atseml]

Force-pushes re: metadata like commit message changes and authorship.