powerpc/pmac: Fix internal modem IRQ on Wallstreet PowerBook

The (relatively) new pmac_zilog driver doesn't use the pre-munged
IRQ numbers from the macio_dev unlike other macio things, it
directly maps it off the OF device-tree.

It does that because it can be initialized much earlier than the
registration of the macio devices, in order to get a serial
console early.

Unfortunately, that means that it "misses" some fixups done
by the macio layer to work around missing interrupt descriptions
in the device-tree of the Wallstreet machines.

This patch brings the necessary workaround into the pmac_zilog
driver itself to bring it back to working condition.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>