Cleanup pcie code

Remove unused variable pcieService, fix pciePath and use global
variables pciePath and pcieDeviceInterface.

Tested: tested PCIeDeviceCollection and PCIeDevice, and tested
the changes with the peci-pcie commit
https://gerrit.openbmc.org/c/openbmc/peci-pcie/+/62100



Change-Id: Iff8aee3f8bc43740b1885a2da584b0e6cf579dcc
Signed-off-by: Lakshmi Yadlapati <lakshmiy@us.ibm.com>