efi: efivars: prevent double registration

Add the missing sanity check to efivars_register() so that it is no
longer possible to override an already registered set of efivar ops
(without first deregistering them).

This can help debug initialisation ordering issues where drivers have so
far unknowingly been relying on overriding the generic ops.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>