Fix vep_vector_number ioread width to follow upstream

In response to below commit which was merged to upstream for v5.6
"PCI/switchtec: Fix vep_vector_number ioread width"
torvalds/linux@9375646

Author: kelvin-cao

switchtec.c

@@ -1497,7 +1497,7 @@ static int switchtec_init_isr(struct switchtec_dev *stdev)
 	if (nvecs < 0)
 		return nvecs;
 
-	event_irq = ioread32(&stdev->mmio_part_cfg->vep_vector_number);
+	event_irq = ioread16(&stdev->mmio_part_cfg->vep_vector_number);
 	if (event_irq < 0 || event_irq >= nvecs)
 		return -EFAULT;