wifi-scripts: ucode: add gcmp256 option, default GCMP-256 per WPA3 mode

wifi-scripts: ucode: add gcmp256 option, default GCMP-256 per WPA3 mode

Commit 1f86f4e471f8 ("wifi-scripts: ucode: simplify wpa_pairwise default
selection") made HE and EHT BSSes default their pairwise cipher to
"GCMP-256 CCMP", and commit 86b9eec8f02b ("wifi-scripts: ucode: add
WPA3-Personal Compatibility Mode") always put GCMP-256 into the RSNE
Override 2 element on EHT compatibility-mode BSSes.

WPA3 Specification v3.5 only makes GCMP-256 mandatory when the BSS enables
EHT or MLO (section 2.5, item 5); for HE and below the WPA3 and Wi-Fi
Enhanced Open Deployment Guide v1.1 lists it as recommended, not required.
Advertising it by default causes interoperability problems: several
clients fail to associate when GCMP-256 is offered as a pairwise cipher
and connect again with CCMP only (Nanoleaf devices, a Motorola/Unisoc
phone, a Linux/iwd laptop).

Add a gcmp256 UCI option and, like sae_ext_key, default it on only where
GCMP-256 is both mandatory and safe: on Compatibility mode (sae-compat)
BSSes running an EHT htmode, which carry it in a separate RSNE Override 2
element that legacy clients ignore. It defaults off for WPA3-Personal
(sae) and Transition (sae-mixed) mode and on non-EHT BSSes. An explicit
'option gcmp256 0/1' overrides the default per BSS.

Both GCMP-256 pairwise defaults now key off config.gcmp256 (and, from the
earlier driver-support change, the phy actually implementing the cipher):
the sae/sae-mixed "GCMP-256 CCMP" pairwise cipher and the sae-compat RSNE
Override 2 element. The 'encryption sae+gcmp256' suffix and the wpa3-192
mode still force GCMP-256 as before.

Fixes: 1f86f4e471f8 ("wifi-scripts: ucode: simplify wpa_pairwise default selection")
Fixes: 86b9eec8f02b ("wifi-scripts: ucode: add WPA3-Personal Compatibility Mode")
Assisted-by: Claude:claude-opus-4-8
Link: https://github.com/openwrt/openwrt/pull/24041
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>