net/mlx5: Allow 0 for total host VFs

When querying eswitch functions 0 is a valid number of host VFs. After
introducing ARM SRIOV falling through to getting the max value from PCI
results in using the total VFs allowed on the ARM for the host.

Fixes: 86eec50b

 ("net/mlx5: Support querying max VFs from device");
Signed-off-by: Daniel Jurgens <danielj@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>