mac80211: mwifiex: replace one-element arrays with flexible array members

mac80211: 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.

This fix has been accepted upstream:
https://github.com/torvalds/linux/commit/1cb5845a58d8e1f85d5766c6fbcbfddf96c212a1

Tested-on: WRT3200ACM
Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24451
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>