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>

it85spi.c

@@ -327,7 +327,7 @@ static int it85xx_spi_send_command(struct flashctx *flash,
 				   const unsigned char *writearr,
 				   unsigned char *readarr)
 {
-	int i;
+	unsigned int i;
 
 	it85xx_enter_scratch_rom();
 	/* Exit scratch ROM ONLY when programmer shuts down. Otherwise, the