USB: Sierra - Add support for Aircard 881U

Adds the appropriate vendor and device IDs for the AirCard 881U to
sierra.c. (This device is often rebadged by AT&T as the USBConnect 881).

Signed-off-by: Jessica L Blank <j@twu.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>