USB: Add PID for Sierra 250U to drivers/usb/serial/sierra.c

Add VID/PID for Sierra Wireless 250U USB dongle to sierra.c
Allows use of 3G radio only

Signed-off-by: August Huber <gus@pbx.org>
Cc: Elina Pasheva <epasheva@sierrawireless.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>