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/en29lv640b.c b/en29lv640b.c
index 550642d..5b01904 100644
--- a/en29lv640b.c
+++ b/en29lv640b.c
@@ -29,7 +29,7 @@
 /* chunksize is 1 */
 int write_en29lv640b(struct flashctx *flash, const uint8_t *src, unsigned int start, unsigned int len)
 {
-	int i;
+	unsigned int i;
 	chipaddr bios = flash->virtual_memory;
 	chipaddr dst = flash->virtual_memory + start;