Bluetooth: use flexible-array member instead of zero-length array

Fix the following coccicheck warning:

net/bluetooth/msft.c:37:6-13: WARNING use flexible-array member instead
net/bluetooth/msft.c:42:6-10: WARNING use flexible-array member instead
net/bluetooth/msft.c:52:6-10: WARNING use flexible-array member instead

Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>