Correct SPL uses of FASTBOOT_FLASH_NAND

This converts 3 usages of this option to the non-SPL form, since there is
no SPL_FASTBOOT_FLASH_NAND defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/drivers/fastboot/fb_getvar.c b/drivers/fastboot/fb_getvar.c
index de4879d..dd3475e 100644
--- a/drivers/fastboot/fb_getvar.c
+++ b/drivers/fastboot/fb_getvar.c
@@ -104,7 +104,7 @@
 					       response);
 		if (r >= 0 && size)
 			*size = disk_part.size * disk_part.blksz;
-	} else if (CONFIG_IS_ENABLED(FASTBOOT_FLASH_NAND)) {
+	} else if (IS_ENABLED(CONFIG_FASTBOOT_FLASH_NAND)) {
 		r = fastboot_nand_get_part_info(part_name, &part_info, response);
 		if (r >= 0 && size)
 			*size = part_info->size;