spl: mmc: get rid of emmc boot code duplication

Get rid of emmc boot code duplication in spl_mmc_load_image() using a switch
case fallthrough into MMCSD_MODE_RAW. Since the #ifdef CONFIG_SUPPORT_EMMC_BOOT
check is not really necessary, remove it in the process.

No functional changes.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>