PCI: Fix PCI Express Capability accessors for PCI_EXP_FLAGS

PCI_EXP_FLAGS_TYPE is a mask, not an offset.  Fix it.

Previously, pcie_capability_read_word(..., PCI_EXP_FLAGS, ...) would
fail.

[bhelgaas:  tweak changelog]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.7+