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.