Correct SPL uses of FASTBOOT_FLASH

This converts 3 usages of this option to the non-SPL form, since there is
no SPL_FASTBOOT_FLASH 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 2fbd285..bcef8f8 100644
--- a/drivers/fastboot/fb_getvar.c
+++ b/drivers/fastboot/fb_getvar.c
@@ -57,7 +57,7 @@
 	}, {
 		.variable = "current-slot",
 		.dispatch = getvar_current_slot
-#if CONFIG_IS_ENABLED(FASTBOOT_FLASH)
+#if IS_ENABLED(CONFIG_FASTBOOT_FLASH)
 	}, {
 		.variable = "has-slot",
 		.dispatch = getvar_has_slot
@@ -67,7 +67,7 @@
 		.variable = "partition-type",
 		.dispatch = getvar_partition_type
 #endif
-#if CONFIG_IS_ENABLED(FASTBOOT_FLASH)
+#if IS_ENABLED(CONFIG_FASTBOOT_FLASH)
 	}, {
 		.variable = "partition-size",
 		.dispatch = getvar_partition_size