[NETFILTER] PPTP helper: Fix endianness bug in GRE key / CallID NAT

This endianness bug slipped through while changing the 'gre.key' field in the
conntrack tuple from 32bit to 16bit.

None of my tests caught the problem, since the linux pptp client always has
'0' as call id / gre key.  Only windows clients actually trigger the bug.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>