net/vdev_netvsc: fix device detection error check

net/vdev_netvsc: fix device detection error check

The vdev_netvsc driver does periodic detection of PCI devices matched
to the netvsc existed interfaces.

When it finds a match, the PCI address is written to the pipe of the
associated fail-safe PMD instance and a positive value is returned to
the periodic check which is wrongly considered as error.

Change the check to consider only a negative value as error.

Fixes: e7dc5d7becc5 ("net/vdev_netvsc: implement core functionality")
Signed-off-by: Matan Azrad <matan@mellanox.com>