wifi: at76c50x: use sizeof(*pointer) instead of sizeof(type)

wifi: at76c50x: 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). This patch has no effect
on runtime behavior.

At the same time remove some redundant NULL initializations.

Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/AS8PR02MB7237C784C14DBC943CB719F88BFE2@AS8PR02MB7237.eurprd02.prod.outlook.com