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>

net/fastboot.c

@@ -307,7 +307,7 @@ void fastboot_start_server(void)
 
 	fastboot_our_port = CONFIG_UDP_FUNCTION_FASTBOOT_PORT;
 
-	if (CONFIG_IS_ENABLED(FASTBOOT_FLASH))
+	if (IS_ENABLED(CONFIG_FASTBOOT_FLASH))
 		fastboot_set_progress_callback(fastboot_timed_send_info);
 
 	net_set_udp_handler(fastboot_handler);