drivers: bluetooth: slz_hci: Set Number of Completed Packets threshold

Configure the Number of Completed Packets event threshold to 1, so the
SiLabs Bluetooth controller will always send the Number of Completed
Packets HCI event to the host, even for small numbers of transmitted
packets.

Fixes #62279

Signed-off-by: Markus Fuchs <markus.fuchs@ch.sauter-bc.com>