spl: mmc: Fix raw boot mode (related to commit 4c5bbc2328a24f5e1ee990c9a9527e48e5fb3b5f)

spl: mmc: Fix raw boot mode (related to commit 4c5bbc2328a24f5e1ee990c9a9527e48e5fb3b5f)

As reported by Robert Nelson, commit 4c5bbc2328a24f5e1ee990c9a9527e48e5fb3b5f
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>