iio: event: use sizeof(*pointer) instead of sizeof(type)

It is preferred to use sizeof(*pointer) instead of sizeof(type)
due to the type of the variable can change and one needs not
change the former (unlike the latter). At the same time refactor
the NULL comparison.

This patch has no effect on runtime behavior.
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Link: https://lore.kernel.org/r/AS8PR02MB7237D024459C314CECE72EAF8BFE2@AS8PR02MB7237.eurprd02.prod.outlook.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-event.c

@@ -581,8 +581,8 @@ int iio_device_register_eventset(struct iio_dev *indio_dev)
 	      iio_check_for_dynamic_events(indio_dev)))
 		return 0;
 
-	ev_int = kzalloc(sizeof(struct iio_event_interface), GFP_KERNEL);
-	if (ev_int == NULL)
+	ev_int = kzalloc(sizeof(*ev_int), GFP_KERNEL);
+	if (!ev_int)
 		return -ENOMEM;
 
 	iio_dev_opaque->event_interface = ev_int;