bootstage: Fix unstash of records from SPL

The commit b81e31a1 ("bootstash: Do not provide a default address
for all") changed a bootstage unstash call to bootstage stash, this
has resulted in bootstage records stashed in SPL no longer get unstaged
in U-Boot proper. Fix this by changing back to a unstage call.

Fixes: b81e31a1 ("bootstash: Do not provide a default address for all")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>