[NETFILTER]: ip_nat_tftp: Fix expectation NAT

When a TFTP client is SNATed so that the port is also changed, the
port is never changed back for the expected connection.

Signed-off-by: Marcus Sundberg <marcus@ingate.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>