spl: mmc: Fix raw boot mode (related to commit 4c5bbc23)

As reported by Robert Nelson, commit 4c5bbc23
may break MMC RAW boot mode.
This patch fixes the check path to fix MMC Raw boot mode.

Tested raw boot mode and FS boot mode on a pandaboard (rev. A3).

Reported-by: Robert Nelson <robertcnelson@gmail.com>
Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Tom Rini <trini@ti.com>
Cc: Robert Nelson <robertcnelson@gmail.com>
Tested-by: Robert Nelson <robertcnelson@gmail.com>