remoteproc: rename len of rpoc_vring to num

Rename the member len in the structure rpoc_vring to num. And remove 'in
bytes' from the comment of it. This is misleading. Because this actually
refers to the size of the virtio vring to be created. The unit is not
bytes.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Message-Id: <20220624025621.128843-2-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>