btm_iso: Fix couple of asserts

LOG_ASSERT we are using to make sure that API is called properly.
This patch removes couple of asserts which are in the places where
stack gets response from the controller.
It is because it might happen that on such events iso is already gone
because of the disconnection, therefor we should handle it gracefully.

Bug: 159786353
Tag: #feature
Test: atest --host net_test_btm_iso
Sponsor: jpawlowski@

Change-Id: I6ff8b007f115fd0289a2afa8ba173daf9b70977a