wifi: mt76: Avoid multiple -Wflex-array-member-not-at-end warnings

wifi: mt76: Avoid multiple -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

So, in order to avoid ending up with a flexible-array member in the
middle of multiple other structs, we use the `struct_group_tagged()`
helper to create a new tagged `struct mt76_connac2_mcu_rxd_hdr`.
This structure groups together all the members of the flexible
`struct mt76_connac2_mcu_rxd` except the flexible array.

As a result, the array is effectively separated from the rest of the
members without modifying the memory layout of the flexible structure.
We then change the type of the middle struct members currently causing
trouble from `struct mt76_connac2_mcu_rxd` to `struct
mt76_connac2_mcu_rxd_hdr`.

We also want to ensure that when new members need to be added to the
flexible structure, they are always included within the newly created
tagged struct. For this, we use `static_assert()`. This ensures that the
memory layout for both the flexible structure and the new tagged struct
is the same after any changes.

This approach avoids having to implement `struct mt76_connac2_mcu_rxd_hdr`
as a completely separate structure, thus preventing having to maintain
two independent but basically identical structures, closing the door
to potential bugs in the future.

So, with these changes, fix the following warnings:

drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:32:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:40:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:49:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:58:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/Zr5KsZugaEXrApQJ@elsanto
Signed-off-by: Felix Fietkau <nbd@nbd.name>