pptp: fix byte order warnings

Pptp driver has lots of byte order warnings from sparse.
This was because the on-the-wire header is in network byte order (obviously)
but the definition did not reflect that.

Also, the address structure to user space actually put the call id
in host order. Rather than break ABI compatibility, just acknowledge
the existing design.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>