eatt: Adjust to new L2CAP API behaviour

There were a changes in the L2CAP layer which needs to be taken into
account by EATT.

L2CAP calls L2CA_DisconnectInd_Cb only when channel
has been disconnected by peer and there is also no confirmation on
L2CA_DisconnectReq.

Having above in mind, EATT needs to clear its channels list just
after calling L2CA_DisconnectReq() othwerwise state of EATT will
be incorrect.

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

Change-Id: I49949ca071f40abb9ca38dcd337b4de63f493f09