Fix -Wsign-compare trouble

Mostly by changing to `unsigned` types where applicable, sometimes
`signed` types, and casting as a last resort.

Change-Id: I08895543ffb7a48058bcf91ef6500ca113f2d305
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/30409
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>
diff --git a/board_enable.c b/board_enable.c
index 5876036..74612f5 100644
--- a/board_enable.c
+++ b/board_enable.c
@@ -373,7 +373,8 @@
 
 static const struct winbond_chip *winbond_superio_chipdef(void)
 {
-	int i, j;
+	int i;
+	unsigned int j;
 
 	for (i = 0; i < superio_count; i++) {
 		if (superios[i].vendor != SUPERIO_VENDOR_WINBOND)