powerpc/rtasd: Don't start event scan if scan rate is zero

There appear to be Pegasos systems which have the rtas-event-scan
RTAS tokens, but on which the event scan always fails. They also
have an event-scan-rate property containing 0, which means call
event scan 0 times per minute.

So interpret a scan rate of 0 to mean don't scan at all. This fixes
the problem on the Pegasos machines and makes sense as well.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/rtasd.c

@@ -490,6 +490,12 @@ static int __init rtas_init(void)
 		return -ENODEV;
 	}
 
+	if (!rtas_event_scan_rate) {
+		/* Broken firmware: take a rate of zero to mean don't scan */
+		printk(KERN_DEBUG "rtasd: scan rate is 0, not scanning\n");
+		return 0;
+	}
+
 	/* Make room for the sequence number */
 	rtas_error_log_max = rtas_get_error_log_max();
 	rtas_error_log_buffer_max = rtas_error_log_max + sizeof(int);