Bluetooth: virtio_bt: fix memory leak in virtbt_rx_handle()

On the reception of packets with an invalid packet type, the memory of
the allocated socket buffers is never freed. Add a default case that frees
these to avoid a memory leak.

Fixes: afd2daa2

 ("Bluetooth: Add support for virtio transport driver")
Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>