USB: cdc-acm: fix runtime PM for control messages

commit bae3f4c53585e9a170da9436e0f06919874bda9a upstream.

Fix runtime PM handling of control messages by adding the required PM
counter operations.

Fixes: 11ea859d ("USB: additional power savings for cdc-acm devices
that support remote wakeup")

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>