USB: serial: omninet: update driver description

With the inclusion of Omni 56K Plus, this driver seem to be more common
among the family of Zyxel omni modem. Update the driver and module
descriptions.

Signed-off-by: Alexandre GRIVEAUX <agriveaux@deutnet.info>
[ johan: amend commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>