Fixed one bug in PciBus. PciBus doesn't clear the bridges bus number for all the root bridges before scanning any of them.
[Impaction]:
Pci Bus driver.
[Reference Info]:
EDK tracker 997.
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4851 6f19259b-4bc3-4df7-8a09-765794883524