Clean up warnings in lk code

Change-Id: I05b3a6cf07cde7e60586b1816c2f1279f64f5cf4
diff --git a/app/aboot/fastboot.c b/app/aboot/fastboot.c
index 6752dde..d247918 100644
--- a/app/aboot/fastboot.c
+++ b/app/aboot/fastboot.c
@@ -258,7 +258,7 @@
 		return;
 
 	r = usb_read(download_base, len);
-	if ((r < 0) || (r != len)) {
+	if ((r < 0) || ((unsigned) r != len)) {
 		fastboot_state = STATE_ERROR;
 		return;
 	}