Bluetooth: conn: Fix tx on iso broadcaster only builds

If CONFIG_BT_ISO_BROADCASTER=y and CONFIG_BT_CONN=n, i.e
an ISO broadcaster only build, then the handling of the
TX callback was not properly initialized, causes a fatal
issue when sending.

This commit moves the tx_complete_work out from the ACL group
and into the common area, so that it will be enabled
for connected ISO as well as broadcaster ISO.

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