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/pickit2_spi.c b/pickit2_spi.c
index 52021d9..784dc13 100644
--- a/pickit2_spi.c
+++ b/pickit2_spi.c
@@ -209,7 +209,7 @@
 	}
 
 	uint8_t buf[CMD_LENGTH] = {CMD_DOWNLOAD_DATA, writecnt};
-	int i = 2;
+	unsigned int i = 2;
 	for (; i < writecnt + 2; i++) {
 		buf[i] = writearr[i - 2];
 	}