Add PCIE_LNKCAP2_SLS2SPEED macro for transforming raw Link Capabilities 2
values to the pci_bus_speed. This is next to PCIE_SPEED2MBS_ENC() to make
it easier to update both places when adding support for new speeds.

Link: https://lore.kernel.org/r/1581937984-40353-10-git-send-email-yangyicong@hisilicon.comSigned-off-by: NYicong Yang <yangyicong@hisilicon.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>