btm_iso: Relax iso_handle check for read link quality function

Since this is rather debug function, and might be called
from some loop without a context of the connected iso's lets allow
for invalid iso_handle to be pass in that function. In case it is incorrect,
just drop the user request.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
Test: atest --host net_test_btm_iso

Change-Id: I658a10d0afbc628b6de656839dfbcf0e47ed244c