net: fastboot: Fix build when FASTBOOT_FLASH is disabled

When building without FASTBOOT_FLASH we don't include the intermediate
update callback to keep the client alive, so ensure we don't try setting
it here.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

net/fastboot.c

@@ -309,7 +309,9 @@ void fastboot_start_server(void)
 
 	fastboot_our_port = WELL_KNOWN_PORT;
 
+#if CONFIG_IS_ENABLED(FASTBOOT_FLASH)
 	fastboot_set_progress_callback(fastboot_timed_send_info);
+#endif
 	net_set_udp_handler(fastboot_handler);
 
 	/* zero out server ether in case the server ip has changed */