In the cyrix Cx5530, there are null (empty) Base address registers before the

In the cyrix Cx5530, there are null (empty) Base address registers before the
base register that controls Ultra-DMA, so we need to examine all possible
base registers instead of just giving up at the first empty one.
Also, looking at the source code to the BIOS, I see that they are also
checking for 0xffffffff as an invalid value so do the same. Stefan may like
to clean this up, but at least now I can find my PCI IDE registers.