Bluetooth: btmtksdio, btmtkuart: validate WMT event length before struct access

btmtksdio.c and btmtkuart.c cast a received WMT event straight to
struct btmtk_hci_wmt_evt and read its op/flag fields without checking
the event is long enough to contain them, unlike btmtk.c. The
FUNC_CTRL case then further casts to struct btmtk_hci_wmt_evt_funcc
and reads its 2-byte status field, again without a length check.
Firmware that sends a short or malformed WMT event makes both drivers
read past the end of the received SKB.

Mirror btmtk.c: validate the base WMT header with skb_pull_data()
before touching any of its fields, and when a FUNC_CTRL event turns
out to be the short, header-only form (a plain enable/disable ack
with no status word), decode the result from the header's own flag
byte instead (0 = success, otherwise failure).

Verified setup on MT7920, MT7921, MT7922 and MT7925: no regression.

Fixes: 9aebfd4a2200 ("Bluetooth: mediatek: add support for MediaTek MT7663S and MT7668S SDIO devices")
Fixes: e0b67035a90b ("Bluetooth: mediatek: update the common setup between MT7622 and other devices")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>