USB: keyspan: fix typo causing GPF on open

Commit f79b2d0f (USB: keyspan: fix NULL-pointer dereferences and
memory leaks) had a small typo which made the driver use wrong
offsets when mapping serial port private data.  This results in
in a GPF when the port is opened.

Reported-by: Richard <richjunk@pacbell.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>