CONFIG_SPL_FAT_SUPPORT was removed in commit 0c3a9ed409
("spl: Kconfig: Replace CONFIG_SPL_FAT_SUPPORT with CONFIG_SPL_FS_FAT").
Fixup a leftover use of the symbol.
Fixes: 9d86dbd9cf ("imx: spl: implement spl_boot_mode for i.MX7/8/8M")
Signed-off-by: Harald Seiler <hws@denx.de>