PatchPanel: Fix crash when tearing down "pass thru" software patch

The pass thru software patch is normally used with the MSD module.

There is no need to establish a peer reference from PatchRecord
because its I/O code only gets executed by PatchTrack
initiative. In fact, this also means PassthroughPatchRecord
can't be stopped synchronously, so the usual software patch tear
down logic in PatchPanel leads to a null pointer dereference
because it races with PatchTrack I/O activity on a separate
thread.

Fix by skipping the sp<> setup / clear logic in PatchPanel
for PassthroughPatchRecord.

Bug: 147599144
Test: force teardown of MSD patch, check logcat for crash
Change-Id: I4a2abc16ad705244767b33ea529e1ace2213d19f