net/mlx5: Support disabling host PFs

Some devices support disabling the physical function on the host. When
this is configured the vports for the host functions do not exist.

This patch checks if host functions are enabled before trying to access
their vports.

Signed-off-by: Daniel Jurgens <danielj@nvidia.com>
Reviewed-by: William Tu <witu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1755112796-467444-3-git-send-email-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>