ichsmb: defer smbus attach until interrupts are available

This fixes a "timed sleep before timers are working" panic seen
while attaching jedec_dimm(4) instances too early in the boot.

Submitted by:	ian
Reviewed by:	hselasky
Differential Revision:	https://reviews.freebsd.org/D21452

sys/dev/ichsmb/ichsmb.c

@@ -131,11 +131,8 @@ ichsmb_attach(device_t dev)
 		goto fail;
 	}
 
-	/* Attach "smbus" child */
-	if ((error = bus_generic_attach(dev)) != 0) {
-		device_printf(dev, "failed to attach child: %d\n", error);
-		goto fail;
-	}
+	/* Probe and attach the smbus when interrupts are available */
+	config_intrhook_oneshot((ich_func_t)bus_generic_attach, dev);
 
 	return (0);