On some DesignWare PCIe controllers accessing config registers of slots
whose link is down triggers a SError. Because of that we need to check the
link status before any acceses config space.
Due to a typo link was always reported up.
This fixes a SError that occured during boot on LS1028A-RDB.
Obtained from: Semihalf
Reviewed by: wma
Differential revision: https://reviews.freebsd.org/D31509
df9c0e88e1