[NETFILTER]: PPTP conntrack: fix PPTP_IN_CALL message types

Fix incorrectly used message types and call IDs:

- PPTP_IN_CALL_REQUEST (PAC->PNS) contains a PptpInCallRequest (icreq)
  message and the PAC call ID

- PPTP_IN_CALL_REPLY (PNS->PAC) contains a PptpInCallReply (icack)
  message and the PNS call ID

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>