[build] Fix uses of literal 0 as a NULL pointer

Detected using sparse.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/linux_args.c

@@ -45,9 +45,9 @@ __asmcall void save_args(int argc, char **argv)
 
 /** Supported command-line options */
 static struct option options[] = {
-	{"net", 1, 0, 'n'},
-	{"settings", 1, 0, 's'},
-	{0, 0, 0, 0}
+	{"net", 1, NULL, 'n'},
+	{"settings", 1, NULL, 's'},
+	{NULL, 0, NULL, 0}
 };
 
 /**