Input: applespi - avoid efivars API and invoke EFI services directly

This driver abuses the efivar API, by using a few of its helpers on
entries that were not instantiated by the API itself. This is a problem
as future cleanup work on efivars is complicated by this.

So let's just switch to the get/set variable runtime wrappers directly.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>