Bluetooth: hci_uart: fix GPF in h5_recv

Syzbot hit general protection fault in h5_recv(). The problem was in
missing NULL check.

hu->serdev can be NULL and we cannot blindly pass &serdev->dev
somewhere, since it can cause GPF.

Fixes: d9dd833c

 ("Bluetooth: hci_h5: Add runtime suspend")
Reported-and-tested-by:  <syzbot+7d41312fe3f123a6f605@syzkaller.appspotmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>