USB: serial: omninet: use generic write implementation

USB: serial: omninet: use generic write implementation

Now that the endpoint-port mapping has been properly set up during
probe, we can switch to using the more efficient generic write
implementation.

Note that this currently means that chars_in_buffer now overcounts
slightly as we always write a full endpoint-sized packet.

Also add a copyright entry.

Signed-off-by: Johan Hovold <johan@kernel.org>