spl: Fix SPL EXT support

Commit 9f12cd0e has broken SPL EXT support.
This patch update error code check to get SPL EXT support working again.

Tested on a Pandaboard (rev. A3).

Reviewed-by: Suriyan Ramasami <suriyan.r@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Tom Rini <trini@ti.com>