device-drivers: pci: fix PCI_EXP_CAP_CONFIG test-case

There is another way to check that PCI Express config space of pci devices
can be read successfully.
Firstly, find out if a device has a PCI Express Capability: we should get
a correct config address offset from the dev's structure (dev->pcie_cap).
Using the offset, read a PCI Express header. Check if we can get the right
PCI Express CAP ID from the header (it must match the PCI_CAP_ID_EXP macro).

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>