wifi: mwifiex: replace one-element arrays with flexible array members

wifi: mwifiex: replace one-element arrays with flexible array members

Replace deprecated one-element arrays with flexible array members.
CONFIG_FORTIFY_SOURCE reports the following warning when
one-element arrays are used as variable-length buffers:

sta_cmd.c:1033 mwifiex_sta_prepare_cmd
memcpy: detected field-spanning write (size 84) of single field
"domain->triplet" at .../marvell/mwifiex/sta_cmd.c:1033 (size 3)

Convert affected structs to use flexible array members.
- Preserve existing wire layouts.
- Use DECLARE_FLEX_ARRAY() for structs inside affected unions.

Tested-on: WRT3200ACM, OpenWrt
Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://patch.msgid.link/20260716001728.57799-1-gvalkov@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>