MFC r351514: Don't consider PCIe hot-plug command timeout fatal.

According to my tests and errata to several generations of Intel CPUs,
PCIe hot-plug command completion reporting is not very reliable thing.
At least on my Supermicro X11DPi-NT board I never saw it reported.
Before this change timeout code detached devices and tried to disable
the slot, that in my case resulted in hot-plugged device being detached
just a second after it was successfully detected and attached. This
change removes that, so in case of timeout it just prints the error and
continue operation. Linux does the same.