PCI: Tidy bitmasks and spacing of PCIe capability definitions

The convention of showing bits in a mask of the full register width, e.g.,
"0x00000007" instead of "0x07" for a field in a 32-bit register, is common
but not universal in this file.  This patch makes it consistently used at
least for the PCIe capability.

Whitespace and zero-extension changes only; no functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>