USB: serial: allow hung up ports to be suspended

User space can keep a tty open indefinitely and that should not prevent
a hung up port and its USB device from being runtime suspended.

Fix this by incrementing the PM usage counter when the port it activated
and decrementing the counter when the port is shutdown rather than when
the tty is installed and the last reference is dropped, respectively.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>