ikev2: Don't increase expected MID after handling MID sync message

ikev2: Don't increase expected MID after handling MID sync message

If the responder never sent a message the expected MID is 0.  While
the sent MID (M1) SHOULD be increased beyond the known value, it's
not necessarily the case.
Since M2 - 1 would then equal UINT_MAX setting that MID would get ignored
and while we'd return 0 in the notify we'd actually expect 1 afterwards.