Bluetooth: hci_h5: close serdev device and free hu in h5_close

When h5_close() gets called, the memory allocated for the hu gets
freed only if hu->serdev doesn't exist. This leads to a memory leak.
So when h5_close() is requested, close the serdev device instance and
free the memory allocated to the hu entirely instead.

Fixes: https://syzkaller.appspot.com/bug?extid=6ce141c55b2f7aafd1c4


Reported-by:  <syzbot+6ce141c55b2f7aafd1c4@syzkaller.appspotmail.com>
Tested-by:  <syzbot+6ce141c55b2f7aafd1c4@syzkaller.appspotmail.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>