btm_iso: Fix race condition

If btm iso is closed when controller is doing some operation for it, the
returning callback will not have valid `this`.
Since we are using single instance of the iso_impl, let introduce guard
which make sure operations are done on valid object.

Bug: 285482385
Test: compile
Tag: #feature
Change-Id: I9afd1ddc125ee2656f2502baa1ce367d3b3c1319