iwlwifi: avoid void pointer arithmetic

Avoid void pointer arithmetic since it's technically
undefined and causes warnings in some places that use
our code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220128153014.e349104ecd94.Iadc937f475158b9437becdfefb361a97e7eaa934@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>