USB: sisusbvga: Fix bug

drivers/usb/misc/sisusbvga/sisusb.c: In function sisusb_open
drivers/usb/misc/sisusbvga/sisusb.c:2444: warning: sisusb is used uninitialized in this function

is a genuine bug (which will cause oops). We cannot use "sisusb" in
error path for (!interface), because sisusb will itself be derived
from "interface" later.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>