Bluetooth: Host: Ensure conn_ready access is thread safe

* The `bt_dev.le.conn_ready` list is accessed by the tx_processor
which runs in a workqueue, but `bt_conn_data_ready` can be called
from different threads so we need to make sure that nothing will
trigger a context switch while we are manipulating the list since
sys_slist_*() functions are not thread safe.
* This only happens if call to `bt_conn_data_ready` is performed
from a preemptive task which can happen depending on the
application.

Signed-off-by: Yago Fontoura do Rosario <yafo@demant.com>