bus/vdev: fix error log on secondary device scan

bus/vdev: fix error log on secondary device scan

When a secondary process handles VDEV_SCAN_ONE mp action, it is possible
the device is already be inserted. This happens when we have multiple
secondary processes which cause multiple broadcasts from primary during
bus->scan. So we don't need to log any error for -EEXIST.

Bugzilla ID: 84
Fixes: cdb068f031c6 ("bus/vdev: scan by multi-process channel")
Cc: stable@dpdk.org
Reported-by: Gage Eads <gage.eads@intel.com>
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Gage Eads <gage.eads@intel.com>