compat: backport pci_is_pcie and pci_pcie_cap

compat: backport pci_is_pcie and pci_pcie_cap

compat_is_pcie already did the same thing as pci_is_pcie, but this
method is now in kernel so the mainline drivers should use this
function instead of accessing the  member in the struct directly. We
backported this function for older kernels.
pci_pcie_cap is used in rtlwifi so we also need this.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>