Bluetooth: Host: Fix access of uninitialized bt_dev.le.acl_pkts

ISO Synchronized Receiver only builds do not transmit and
hence may not have any tx buffers allocated in a
Controller, leaving bt_dev.le.acl_pkts semaphore
uninitialized or bt_conn_get_pkts() returning NULL.
Do not use the semaphore if no Tx buffers allocated in a
Controller.

Regression in commit ef19c64f ("Bluetooth: host: poll on
CTLR buffers instead of host TX queue").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>