efivars: Parameterize operations.

Instead of letting efivars access struct efi directly when dealing with
variables, use an operations structure.  This allows a later change to
reuse the efivars logic without having to pretend to support everything
in struct efi.

Signed-off-by: Mike Waychison <mikew@google.com>
Cc: Matt Domsch <Matt_Domsch@dell.com>,
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>