efi-pstore: Fix read iter after pstore API refactor

During the internal pstore API refactoring, the EFI vars read entry was
accidentally made to update a stack variable instead of the pstore
private data pointer. This corrects the problem (and removes the now
needless argument).

Fixes: 125cc42b

 ("pstore: Replace arguments for read() API")
Signed-off-by: Kees Cook <keescook@chromium.org>