initialize the MFUNC registers. Our old test of assuming that if this
register is set at all is not quite right. Many scenarios (including
the power-on defaults for chips w/o EEPROMs) land us in trouble. The
MFUNC0 pin should be set to signal #INTA and the MFUNC1 pin should be
set to signal #INTB of multi-socketed devices. Since my memory recalls
issues with blindly clearing the upper bytes of this register, perform
the heuristic only when both MFUNC0 and 1 are clear. We won't work
well using these pins for GPIO, and the serial interrupts won't save
us because we go out of our way to generally disable them. They are
needed to support legacy drivers for 16-bit PC Cards that are
hard-wired to specific IRQ values. Since FreeBSD never had any of
these, we configure the more reliable direct signaling. This was just
one small piece of that which had been left out back in the day.
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