Bluetooth: ISO: Add iso channel state check for unbind

A channel should only be allowed to unbind when in
the BT_ISO_BOUND state. If it is in a disconnected state,
then it the function would have meaning. If it is a connected
state, unbinding it would generate an error when removing
the CIG, which would fail silently.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>