spl: if MMCSD_MODE_RAW fails, try MMCSD_MODE_FS

spl: if MMCSD_MODE_RAW fails, try MMCSD_MODE_FS

Since commit fd61d39970b9901217efc7536d9f3a61b4e1752a:
        spl: mmc: add break statements in spl_mmc_load_image()
RAW and FS boot modes are now exclusive again. So, if MMCSD_MODE_RAW fails, the
board hangs. This patch allows to try MMCSD_MODE_FS then.

It has been tested on a beaglebone black to boot on an EXT partition.

Signed-off-by: Guillaume GARDET <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Nikita Kiryanov <[email protected]>
Cc: Igor Grinberg <[email protected]>
Cc: Paul Kocialkowski <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Matwey V. Kornilov <[email protected]>
Acked-by: Nikita Kiryanov <[email protected]>