env: efi: fix a wrong address dereference

Probably, a pointer to a variable in an inner block should not
be exposed to an outer block.

Fixes: c70f4481 ("efi_loader: simplify 'printenv -e'")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
[trini: Don't make guid const now]
Signed-off-by: Tom Rini <trini@konsulko.com>