btm_iso: Remove assert on CIS connected

Now, when audio data are provided from other thread, we need to accept
that there might be a race when CIS gots disconnected e.g. due to timeout.

Bug: 220039391
Bug: 150670922
Test: atest --host net_test_btm_iso
Change-Id: I7705aad3bc87668deab9b7db416fda6e00e5e7b6