[PATCH 1/1] wifi: mt76: mt7996: fix mixed FEM check of mt7996 chipset

[Valera Kozlov]

Device: ASUS ZenWiFi BT8 (RT-BE14000), MediaTek MT7988D SoC (tri-band,
dual PCIe hif/hif2). Confirmed empirically via extended TX PER testing
that adie_idx=1 is correct for this board (matches Chukun's patch),
consistent with a mixed-FEM design.

Readouts from the ADIE_ID registers:

root at bt8:~# echo 0x0f00002c >/sys/kernel/debug/ieee80211/phy0/mt76/regidx
root at bt8:~# grep . /sys/kernel/debug/ieee80211/phy0/mt76/rf_regval
0x79768a20
root at bt8:~# echo 0x1f00002c >/sys/kernel/debug/ieee80211/phy0/mt76/regidx
root at bt8:~# grep . /sys/kernel/debug/ieee80211/phy0/mt76/rf_regval
0x79778a10
root at bt8:~# echo 0x2f00002c >/sys/kernel/debug/ieee80211/phy0/mt76/regidx
root at bt8:~# grep . /sys/kernel/debug/ieee80211/phy0/mt76/rf_regval
0x79778a10

Note that band1/band2 (0x79778a10) match the EE4600 signature exactly
(needs adie_idx=1), but band0 (0x79768a20) differs from both Zyxel
devices reported so far (both show 0x79758a02 regardless of which
adie_idx they need). This is a third, distinct band0 signature -
suggests band0 alone may not reliably disambiguate the two cases,
since devices needing different adie_idx values can share it.

Happy to test further register dumps or a proposed fix on this
hardware if useful.