USB: pl2303: fix port-data memory leak

Fix port-data memory leak by allocating and freeing port data in
port_probe/remove rather than in attach/release, and by introducing
serial private data to store the device type which is interface rather
than port specific.

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

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