efivars: Use to_efivar_entry

Use to_efivar_entry() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Link: http://lkml.kernel.org/r/1454364428-494-9-git-send-email-matt@codeblueprint.co.uk


Signed-off-by: Ingo Molnar <mingo@kernel.org>