Fix the so-called "half-baked-probe" code that I wrote a long time ago

Fix the so-called "half-baked-probe" code that I wrote a long time ago
to properly clear the interrupt register on the no error case.  Also,
set the mcr register to zero when we find we can't support the chip.

This fixes the hang on sio driver attach problem in the new pci pccard
code that some people have reported.  At least on my machine.  I'd
like to get this into 4.4.

Submitted by: bde
PR: kern/29742

MFC after: 1 day