util/nvmutil: reset mac in main

we currently only run the logic once, but i might
expand nvmutil in the future, so that it can
operate on multiple files. this would require
using a different command syntax, e.g. getop-style
syntax.

this is a preventative bug fix, resetting global
state.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -98,6 +98,8 @@ static void (*cmd)(void) = NULL;
 int
 main(int argc, char *argv[])
 {
+	mac = NULL;
+
 	argv0 = argv[0];
 	if (argc < 2)
 		usage();