USB: digi_acceleport: fix port-data memory leak

Fix port-data memory leak by moving port data allocation and
deallocation to port_probe and port_remove.

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

Note that the oob port is never registered as a port device and should
thus be handled in attach and release.

Compile-only tested.

Cc: Peter Berger <pberger@brimson.com>
Cc: Al Borchers <alborchers@steinerpoint.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>