util/nvmutil: reset errno when writing a MAC

if checksum verification passed, then we should reset
in case we're operating on a given part and the last
one checked was bad.

a catch-all reset is already performed in writeGbe,
but it's good to do it here too.

in practise, if the 2nd part (part 1) is what failed,
errno still wouldn't be reset.

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

util/nvmutil/nvmutil.c

@@ -251,6 +251,7 @@ cmd_setmac(void)
 	for (int partnum = 0; partnum < 2; partnum++) {
 		if (!goodChecksum(part = partnum))
 			continue;
+		errno = 0;
 
 		for (int w = 0; w < 3; w++) /* write MAC to gbe part */
 			setWord(w, partnum, mac[w]);