btm/iso: Set connecting state while issuing establish cis

This patch sets state to connecting while trying to establish CIS. It
is possible to call command which touches CIS for which even command
status did not came. Without this it may lead to assert "Assert failed:
cis->state_flags & kStateFlagIsConnected || cis->state_flags &
kStateFlagIsConnecting. Not connected".

Bug: 208912945
Tag: #feature
Sponsor: jpawlowski@
test: atest --host  net_test_btm_iso
Change-Id: I5fdf94e51d5a638437ddbb64c92b2ccc41f82cbe