Bluetooth: controller: split: Fix Tx FIFO, queue, and pool corruption

Fix the Tx FIFO, queue and pool corruption due to missing
release of link object to Tx link pool and hence missing
reset of the per connection initially allocated Tx link
free pointer.

The bug caused Tx PDUs and associated memory to be lost
leading to missing L2CAP segment transmissions. With lost
control PDU buffers, ULL would stall processing Done events
also leading to controller asserts.

Fixes #18546.

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