Correct SPL uses of FASTBOOT_FLASH_MMC

This converts 5 usages of this option to the non-SPL form, since there is
no SPL_FASTBOOT_FLASH_MMC defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>