ppc/xics: allow ICSState to have an offset 0

commit 15ed653f ("ppc/xics: An ICS with offset 0 is assumed to be
uninitialized") introduced an extra check on the ICS offset which is
not strictly necessary.

Revert the change to be able to map the XICS IRQ number space on the
XIVE IRQ number space.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

include/hw/ppc/xics.h

@@ -139,8 +139,7 @@ struct ICSState {
 
 static inline bool ics_valid_irq(ICSState *ics, uint32_t nr)
 {
-    return (ics->offset != 0) && (nr >= ics->offset)
-        && (nr < (ics->offset + ics->nr_irqs));
+    return (nr >= ics->offset) && (nr < (ics->offset + ics->nr_irqs));
 }
 
 struct ICSIRQState {