USB: keyspan: fix NULL-pointer dereferences and memory leaks

Fix NULL-pointer dereference at release by moving port data allocation
and deallocation to port_probe and port_remove.

Fix NULL-pointer dereference at disconnect by stopping port urbs at
port_remove.

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

Note that this patch also fixes port and interface-data memory leaks in
the error path of attach should port initialisation fail for any port.

Compile-only tested.

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