Bluetooth: btusb: fix excessive stack usage

Enlarging the size of 'struct btmtk_hci_wmt_cmd' makes it no longer
fit on the kernel stack, as seen from this compiler warning:

drivers/bluetooth/btusb.c:3365:12: error: stack frame size of 1036 bytes in function 'btusb_mtk_hci_wmt_sync' [-Werror,-Wframe-larger-than=]

Change the function to dynamically allocate the buffer instead.
As there are other sleeping functions called from the same location,
using GFP_KERNEL should be fine here, and the runtime overhead should
not matter as this is rarely called.

Unfortunately, I could not figure out why the message size is
increased in the previous patch. Using dynamic allocation means
any size is possible now, but there is still a range check that
limits the total size (including the five-byte header) to 255
bytes, so whatever was intended there is now undone.

Fixes: 48c13301 ("Bluetooth: btusb: Fine-tune mt7663 mechanism.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>