ppc/pnv: Fix LPC serirq routing calculation

ppc/pnv: Fix LPC serirq routing calculation

The serirq routing table is split over two registers, the calculation
for the high irqs in the second register did not subtract the irq
offset. This was spotted by Coverity as a shift-by-negative. Fix this
and change the open-coded shifting and masking to use extract32()
function so it's less error-prone.

This went unnoticed because irqs >= 14 are not used in a standard
QEMU/OPAL boot, changing the first QEMU serial-isa irq to 14 to test
does demonstrate serial irqs aren't received, and that this change
fixes that.

Cc: qemu-stable@nongnu.org
Reported-by: Cédric Le Goater <clg@redhat.com>
Resolves: Coverity CID 1558829 (partially)
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>