staging: unisys: visorchannel_write(): Handle partial channel_header writes

visorchannel_write(struct visorchannel *channel, ulong offset,

		   void *local, ulong nbytes)

{

	size_t size = sizeof(struct channel_header);

	size_t chdr_size = sizeof(struct channel_header);

	size_t copy_size;

	if (offset + nbytes > channel->memregion.nbytes)

		return -EIO;

	if (!offset && nbytes >= size)

		memcpy(&channel->chan_hdr, local, size);

	if (offset < chdr_size) {

		copy_size = min(chdr_size, nbytes) - offset;

		memcpy(&channel->chan_hdr + offset, local, copy_size);

	}

	memcpy_toio(channel->memregion.mapped + offset, local, nbytes);