le_audio: Don't trigger connection when device is not disconnected

LE Audio device should not be triggered to connect when device is in
invalid state. Having device in invalid state may cause problems with
setting its state as connecting e.g. when device is already connected.

Bug: 260166914
Tag: #feature
Test: test bluetooth_le_audio_test bluetooth_le_audio_client_test
Change-Id: Ifd29256477651a29e45b980f7398aa9e038aa37b