Bluetooth: controller: split: Fix for Tx Buffer Overflow error

When more than one simultaneous connections are active,
transmitting data packets to peer, a termination causes
host to use the flushed pending number of completed packets
count for other active connections. This is on reception
of HCI disconnection complete event. But the controller has
not yet released any of the pending enqueued Tx buffers
which was happening after the disconnection event was
dispatched to HCI layer.

The fix here is to dispatch the disconnection complete event
from the LLL context after pending Tx buffers have been
flushed and the buffers get returned to Tx pool in the ULL
context. This way buffers are in the Tx pool before host
get to process the disconnection complete event.

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