bus/pci: fix resource mapping override

bus/pci: fix resource mapping override

When scanning an already plugged device, the virtual address
of mapped PCI resource in rte_pci_device will be overridden
with 0, that may cause driver does not work correctly.
The fix is not to update any rte_pci_device's field if the being
scanned device's driver is already probed.

Bugzilla ID: 85
Fixes: c752998b5e2e ("pci: introduce library and driver")
Cc: stable@dpdk.org
Reported-by: Geoffrey Lv <geoffrey.lv@gmail.com>
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>