iwlwifi: Switch to proper EFI variable store interface

Using half of the efivar API with locally baked efivar_entry instances
is not the right way to use this API, and these uses impede planned work
on the efivar layer itself.

So switch to direct EFI variable store accesses: we don't need the
efivar layer anyway.

Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>