USB: option: fix port-data abuse

commit 4273f9878b0a8271df055e3c8f2e7f08c6a4a2f4 upstream.

Commit 8b4c6a3ab596961b78465 ("USB: option: Use generic USB wwan code")
moved option port-data allocation to usb_wwan_startup but still cast the
port data to the old struct...

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>