Bluetooth: hci_h5: fix memory leak in h5_close

When h5_close() is called, h5 is directly freed when !hu->serdev.
However, h5->rx_skb is not freed, which causes a memory leak.

Freeing h5->rx_skb and setting it to NULL, fixes this memory leak.

Fixes: ce945552 ("Bluetooth: hci_h5: Add support for serdev enumerated devices")
Reported-by:  <syzbot+6ce141c55b2f7aafd1c4@syzkaller.appspotmail.com>
Tested-by:  <syzbot+6ce141c55b2f7aafd1c4@syzkaller.appspotmail.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>