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

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")

Cc: <stable@vger.kernel.org>	# v2.6.27
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>