pnpacpi: fix shareable IRQ encode/decode

When we encode IRQ resources, we should use the "shareable" flag we got
from _PRS rather than guessing based on the IRQ trigger mode.

This is based on a patch by Tom Jaeger:
    http://bugzilla.kernel.org/show_bug.cgi?id=9487#c32

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>