[2026_R1] iio: trx-rf: ad9088: fix FFT sniffer ADC select and side select

[mhennerich]

Backport of #3544 to the 2026 R1 release branch (Fixes: tags rewritten to the release branch hash of the initial APOLLO commit).

PR Description

Two fixes for the AD9084/AD9088 FFT spectrum sniffer (ad9088_fft.c).

1. adc_select is silently reverted to ADC0 on every buffer enable

Writing the adc_select attribute programs the sniffer ADC mux immediately,
but never updates sniffer_config.pgm.adc, which keeps its probe-time
default of ADI_APOLLO_ADC_0. adi_apollo_sniffer_pgm() internally calls
adi_apollo_sniffer_adc_mux_set() with pgm.adc, and the driver runs it
from buffer_postenable() on a copy of sniffer_config. So the moment a
capture starts the mux is switched back to ADC0, and selecting adc1 has
no visible effect on the captured spectrum.

Keep pgm.adc in sync with the user selection so the value programmed on
buffer enable matches what was requested.

2. Redundant side_sel assignment in sniffer probe

The switch on side_sel already stores the matching
adi_apollo_sniffer_select_e in st->side_sel; a later unconditional
assignment overwrites it with the adi_apollo_side_select_e parameter.
The two enums share the same values today so this is harmless, but it is
confusing and would silently break if either enum changed. Drop it.

Both commits carry a Fixes: tag pointing at the initial APOLLO support
commit (81b4aed).

Compile-tested for arm64 (adi_zynqmp_defconfig, CONFIG_AD9088=y).

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have compiled my changes, including the documentation
• I have tested the changes on the relevant hardware
• I have updated the documentation outside this repo accordingly
• I have provided links for the relevant upstream lore