ISO broadcast cannot send with callback if CONFIG_BT_CONN=n

[Thalley]

Describe the bug
#38194 introduced a bug for ISO broadcast if CONFIG_BT_CONN=n.
If CONFIG_BT_CONN=n then the free_tx FIFO is never intiialized, causes the FIFO to be empty and thus no able to use the bt_conn_send_cb function that bt_iso_chan_send now relies on.

Update: It seems that TX still works, but the callback is missing.

To Reproduce
Steps to reproduce the behavior:
Build and run samples/bluetooth/iso_broadcast/ and see the <err> bt_conn.bt_conn_send_cb: Unable to allocate TX context error

Expected behavior
Expect broadcast ISO to be fully independent on CONFIG_BT_CONN.

Impact
Showstopper for specific builds. Increased memory usage as a work around.

Logs and console output
N/A

Environment (please complete the following information):

• Commit SHA or Version used: cd5357c

Additional context
Add any other context about the problem here.

[carlescufi]

@Thalley will you send a fix in time for 2.7.0?

[Thalley]

@Thalley will you send a fix in time for 2.7.0?

That's the plan :)

[Thalley]

Updated the priority to low as the callback is just missing, but it is still sending data.