netdevsim: Fix unsigned being compared to less than zero

The comparison of len < 0 is always false because len is a size_t. Fix
this by making len a ssize_t instead.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: d3953819 ("netdevsim: Add max_vfs to bus_dev")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/netdevsim/bus.c

@@ -111,7 +111,7 @@ ssize_t nsim_bus_dev_max_vfs_read(struct file *file,
 {
 	struct nsim_bus_dev *nsim_bus_dev = file->private_data;
 	char buf[11];
-	size_t len;
+	ssize_t len;
 
 	len = snprintf(buf, sizeof(buf), "%u\n", nsim_bus_dev->max_vfs);
 	if (len < 0)