drm/radeon: Replace numbers with PCI_EXP_LNKCTL2 definitions

Replace hard-coded magic numbers with the descriptive PCI_EXP_LNKCTL2
definitions.  No functional change intended.

Link: https://lore.kernel.org/r/20191112173503.176611-4-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>