Bluetooth: hci_serdev: make hci_serdev_client_ops static

The structure hci_serdev_client_ops does not need to be in global scope
and is not modified, so make it static.

Cleans up sparse warning:
"symbol 'hci_serdev_client_ops' was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>