intrng: Store the IPI priority

When registering an interrupt controller we take a priority to compare.
We never store the priority so any later calls may act incorrectly.

Store the priority so later calls act as expected.

Reviewed by:	jrtc27
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D47235

sys/kern/subr_intr.c

@@ -1863,8 +1863,10 @@ intr_ipi_pic_register(device_t dev, u_int priority)
 		return (EBUSY);
 	}
 
-	if (intr_ipi_dev == NULL || priority > intr_ipi_dev_priority)
+	if (intr_ipi_dev == NULL || priority > intr_ipi_dev_priority) {
+		intr_ipi_dev_priority = priority;
 		intr_ipi_dev = dev;
+	}
 
 	return (0);
 }