Bluetooth: ISO: Add check for pending ISO connections

Per the HCI spec it is not possible to send the
HCI_LE_Create_CIS command while an outstanding request
is pending. To avoid failing the command we can
check and verify if any ISO connections are in the
pending state.

Since multiple ISO channels can be connected
in a single request, the solution implemented was
to iterate over all ISO connections and see if there was
any unicast ISO channels in the connecting state.

It also checks for the encryption pending state, as
we should not start the encryption procedure if we know
that the connect ISO command cannot be completed afterwards.

This adds a new return value to bt_iso_chan_connect,
and decided to properly document the return values the
function can return.

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