USB: serial: Fix memory leak in sierra_release()

commit f7bc5051 upstream.

I found a memory leak in sierra_release() (well sierra_probe() I guess)
that looses 8 bytes each time the driver releases a device.

Signed-off-by: Len Sorensen <lsorense@csclub.uwaterloo.ca>
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>