USB: cp210x: fix port-data memory leak

Fix port data memory leak by replacing port private data with serial
private data.

Since commit 0998d063 (device-core: Ensure drvdata = NULL when no
driver is bound) the port private data is no longer freed at
release.

The private data is used to store the control interface number, but as
this is the same for all ports on an interface it should be stored as
usb-serial data anyway.

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