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

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.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>