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: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>