Bluetooth: hci_conn: Use struct_size() in hci_le_big_create_sync()

Use struct_size() instead of the open-coded version. Similarly to
this other patch[1].

Link: https://lore.kernel.org/linux-hardening/ZiwwPmCvU25YzWek@neat/

 [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>