Bluetooth: Conn: Guard tx_complete_work with CONFIG_BT_CONN_TX

Add new Kconfig BT_CONN_TX which is true if the ACL and
ISO configuration allows for sending data. This is mainly
used to avoid initialization of the tx_complete_work
for ISO sync receiver only builds.

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