eal: fix hotplug add and remove

eal: fix hotplug add and remove

If hotplug add an already plugged PCI device, it will
cause rte_pci_device->device.name be corrupted due to unexpected
rte_devargs_remove. Also if try to hotplug remove an already
unplugged device, it will cause segment fault due to unexpected
bus->unplug on a rte_device whose driver is NULL.
The patch fix these issues.

Fixes: 7e8b26650146 ("eal: fix hotplug add / remove")
Cc: stable@dpdk.org
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>