le_audio: Consider desired size of set when picking CIS setup

While getting cis count a desired set size should be considered instead
of connected devices to prepare CIG setup for future connection of
missing device.

Tag: #feature
Bug: 274000386
Test: atest bluetooth_le_audio_test
Change-Id: I778a51922c0e5defd4da42154a23a3f5c8694c30