net/eth: Add an assert() and invert if() statement to simplify code

net/eth: Add an assert() and invert if() statement to simplify code

To simplify the function body, invert the if() statement, returning
earlier.
Since we already checked there is enough data in the iovec buffer,
simply add an assert() call to consume the bytes_read variable.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>