USB: serial: omninet: add device id for Zyxel Omni 56K Plus

Add device id for Zyxel Omni 56K Plus modem, this modem include:

USB chip:
NetChip
NET2888

Main chip:
901041A
F721501APGF

Another modem using the same chips is the Zyxel Omni 56K DUO/NEO,
could be added with the right USB ID.

Signed-off-by: Alexandre GRIVEAUX <agriveaux@deutnet.info>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>