serial/pmac_zilog: Workaround problem due to interrupt on closed port

It seems that in qemu, we can see an interrupt in R3 despite the
fact that it's masked in W1. The chip doesn't actually issue an
interrupt, but we can "see" it when taking an interrupt for the
other channel. This may be a qemu bug ... or not, so let's be
safe and avoid calling into the UART layer when that happens which
woulc cause a crash.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Rob Landley <rob@landley.net>