drivers: bluetooth: Align bt_hci_send behavior on error

This change alignes HCI drivers behavior with Host expectation. That is:
if an HCI driver managed to send a packet to Controller, the HCI driver
also unreferences it. If the HCI driver didn't manage to send the
packet to Controller and returns an error code, it does not unreferences
buffer.

This change aligns the behavior of HCI drivers with the Host's
expectations. Specifically:
- If an HCI driver successfully sends a packet to the Controller, the
  HCI driver also unreferences it.
- If the HCI driver fails to send the packet to the Controller and
  returns an error code, it does not unreference the buffer.

Fixes #94445

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>