ppc: abort if compat property contains an unknown value

It is not possible to set the compat property to an unknown value with
powerpc_set_compat(). Something must have gone terribly wrong in QEMU,
if we detect an "Internal error" in powerpc_get_compat(). Let's abort then.

This patch also drops the "max_compat ? *max_compat : -1" construct. It is
useless since max_compat is dereferenced a few lines above.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>