powerpc Don't print misleading facility name in facility unavailable exception

The current facility_strings[] are correct when the trap address is
0xf80 (hypervisor facility unavailable). When the trap address is
0xf60 (facility unavailable) IC (Interruption Cause) a.k.a status in the
code is undefined for values 0 and 1.

Add a check to prevent printing the (misleading) facility name for IC 0
and 1 when we came in via 0xf60. In all cases, print the actual IC
value, to avoid any confusion.

This hasn't been seen on real hardware, on only qemu which was
misreporting an exception.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
[mpe: Fix indentation, combine printks(), massage change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>